Straight Talk About Oil Filters

 Information provided courtesy of Purolator Filters


Let’s face it. While performance engines represent the most fun in our businesses, it’s the everyday engines we build that pay the rent for most of us. That’s just a fact of life. We can all put valve guides in small-block Chevy heads pretty much blindfolded, and we all know how easily a Honda Accord crank should turn once all the main bearings are torqued up.


But what do we do when it’s time to spin an oil filter onto a newly assembled stock engine? In most cases, it’s probably the equivalent of asking our helper to pick up an oil filter and quickly put it on so we can send the finished job out to the customer.

But are we really doing justice to the customer and, in fact, to our careful work? After all, does it really matter which filter we screw onto a fresh everyday motor? Well, sure it does. That first filter must protect our fresh engine from whatever chips and machining debris might linger in the recesses of the oil galleries and crankcase. And, probably more importantly, the filter we install is likely to establish the precedent for the brand of filter that the car owner will continue to use. So our choice is more long-lived than we might think.

Are all filters the same?

And so we come to the next question: Aren’t all oil filters pretty much the same?

 As it turns out, they’re not.

 For the straight skinny on oil filters for everyday engines we went to the experts at Purolator, since they’re the ones who invented the first automotive oil filter (in 1923!), and also the ones who invented the spin-on oil filter in 1955, as well as a highly respected supplier of OEM filters. Ramon Nuñez, Director of Filtration for Bosch, joint venture owner of Purolator told us that oil filters are deceptively simple and yet deceptively complex.


Here’s the deceptively simple thing about oil filters: the two most important aspects of oil filters are:

What size particles can they filter out?

How much debris can a filter hold before the bypass valve quietly directs unfiltered oil to your new bearings, lifters, and cam lobes?

Sure there are other features to consider, like the integrity of the housing and crimping, the design and reliability of the bypass valve, and the durability of the filtering media itself. But if the filter isn’t safely capturing and storing debris, then the other features don’t matter much.

Purolator’s Nuñez suggests that we first consider particle size. After all, it’s the big chunks we’re worried about, right? Well that’s true if all you’re worried about is the first few hours of operation. But if you’re interested in the longevity of the engine, then size really does matter.

How the filtering media works

Here’s a surprise: the filtering media does not actually filter.

Yup, that’s right. The filtering media does not actually filter. Remember the oil bath air cleaners of the ’40s and ’50s? If you’re old enough to remember them, you may or may not be aware that they worked by forcing incoming air to follow an “S”-shaped route past a trough of engine oil. The concept was, the air could easily follow the turns, while the heavier particles of dirt and debris couldn’t make the turn, and momentum would carry the grit straight into the oil, where it was held hostage until someone dumped the dirty oil and replenished it.

Believe it or not, that’s actually how modern oil filters work. The filtering media does not act like a screen, but rather traps contaminants that cannot change direction easily. Of course, explains Purolator’s Nuñez, today’s oil filter media is very sophisticated, and much science has gone into its design and construction. The design of the media determines how small a particle can be held, and how much capacity the filter has for holding debris.

By the way, Nuñez also notes that the configuration of the size, shape, and number of pleats in a given oil filter are actually mathematically calculated to allow for the maximum amount of media to be exposed to the oil flow.

So back to particle size and filter capacity. Let’s deal with particle size first.

Here’s the short answer: if you can see it, it’s too big, it’ll damage the engine, and it needs to be filtered out.

OK, OK, here’s the long answer. The term “efficiency” is used in the filtration world to describe the amount or percentage of contaminants that are caught and held as oil passes through the filter. There are SAE standards for “test debris” and for testing procedures. Purolator Classic oil filters are 97.5 percent efficient, and Purolator PureONE filters are 99.9 percent% efficient – the highest efficiency ratings on the market. Both filters will essentially remove their respective percentages of particles 20 microns or larger in diameter — including virtually all particles large enough to be visible to the naked eye.

Picture this: a human hair can measure as little as 30 microns (one micron is a millionths of a meter) in diameter (don’t forget we’re talking an end view here…). A human bacteria can measure 20 microns. And the soot found floating around in cigarette smoke can measure nearly one micron. So a filter that removes virtually all particles 20 microns or larger is offering phenomenal protection to your engine.

Here’s more food for thought. A single chip from your rotary broach or milling machine often runs around 0.007”, which translates into about 175 microns. And a stray piece of casting flash is certainly much larger. You can well imagine, and have probably seen, the devastating effects of having a 0.007” piece of debris trying to squeeze itself into the 0.002-0.003” clearance between a rod or main bearing shell and a crankshaft journal. And who among us can claim with certainty that they’ve never left a single, or modest collection of, machining chips hidden away in an engine block or cylinder head?

There are two ways to measure filter efficiency – single-pass and multi-pass. Which test do you think better represents real-world oil filtration – a single-pass test that runs just 45 gallons of oil through a test filter, or a multi-pass test that passes more than 2,500 gallons through the filter? Why it’s the multi-pass test of course.

But won’t a filter that’s more efficient get clogged sooner, causing the bypass valve (if the filter has one…) to open and direct unfiltered oil to the crankshaft, bearings, and other critical components? Ah, that brings us to what the filter engineers refer to as capacity. Capacity represents the amount of contaminants a filter can remove and hold before flow is restricted. Capacity is usually measured in grams.

Some filter makers don’t advertise capacity if it’s not a favorable number for them. In the case of Purolator PureONE oil filters, they have a capacity of at least 13 grams. In real-world terms that means that a PureONE filter will hold the equivalent of 31 standard-size paper clips before it becomes blocked. And that’s a whole lot of debris.

Beyond efficiency and capacity, there are other features you should look for in your choice of an oil filter – things like a steel center tube for reliable support of the media, a one-piece anti-drainback valve (less likely to leak than a multi-piece design), and a flat sealing ring, shown by SAE tests to provide greater sealing surface area, higher blowout resistance, and longer life than O-ring or P-ring designs. Purolator oil filters provide all this and more. Some filter companies use paper or felt end caps versus the steel end caps used by Purolator. 

In cold climates where cold start-ups can cause huge momentary spikes in oil pressure, you should look for a filter design that’s been tested for burst strength. Our Purolator expert tells us that Purolator Classic and PureONE oil filters, for example, are tested 25,000 cycles at pressure pulses of 0-100-0 psi to validate filter housings’ mechanical strength.

If you ever wondered about how an oil filter does it’s job, now you know.

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  • Filterperson

    AMSOIL claims their Ea filters, rated 98.7% efficienct at 15 microns, are the most efficient filters on the market. Purolator says the same about their 99.9% efficient filters at 20 microns. Who's right?

  • http://www.precisionenginetech.com mike.mavrigian

    All filter manufacturers are going to claim high filtration. While I don’t have personal experience with the Amsoil filters, I am very impressed with Purolator. They allow great flow while capturing extremely minute particles. I’ve been using their filters on dyno and the track for about the past 10 years now with no problems.

  • Killa Dave

    vry kul an neet

  • Dmack

    yall gay as fuck

  • Dmack

    killa dave u gay ass fuck boy

  • Rajaun12

    hahah dmack chumpd ya ass off lmao………

  • Larry94037

    Good info

  • http://www.municipalvehicles.org.uk/ Municipal Vehicles

    The higher the velocity goes, the lower the corresponding viscosity should be. The same applies to the bearings.

  • http://www.bes.co.uk/ Plumbing

    On track days, heavy driving and in high performance engines, the heat build up can be extremely high. In this case using an oil with an SAE rating of 50 makes more sense. Oil viscosity is determined by lateral force and speed. In a turbo or crankshaft bearing there is no lateral force, just velocity.

  • Anonymous

       I have (owned) autocrossed and tracked a modified ’00 BMW 328Ci, a modified ’99 BMW M3, and a modified 1990 BMW E30 325is and have had the chance to drive many other cars at these events, including but not limited to Porsche ’77-’09 Porsche 911′s (Carrera/Carrera”S”, Turbo’s, a GT2, and three GT3/GT3RS’s), stock and modified 986/987 Cayman and Boxster “S”s, more BMW’s than I can remember, a Ferrari 360 Challenge Stradale, a Lotus Exige S240, a number of Mercedes AMG cars (C32, C63, CLK63, a few others, and my favorites: the Euro 190E 2.5-16v AMG Power-Pak and the W124 500E), and a number of Audi’s (S4′s, RS4′s, RS2, RS6, A3 S-Line, TT-S, etc) although I think Audi’s are boring and un-challenging.
       My point is, in all of these different cars, whether stock, mildly modified, noticeably modified, or straight-out balls-to-the-wall modified, whether daily drivers or track-only cars, and making anywhere from 150whp to well over 700whp; every single one of them used oil filters from either K&N or Royal Purple, IIRC.
       Now, I’ve always run Royal Purple Oil and Redline Transmission/Differential fluids in my own cars, but I always used the OEM BMW oil filter, as I’d heard that the Purolator filters just weren’t “up to snuff”.  I’ve since been using Royal Purple filters, and they work great, especially with a magnetic drain plug for extra protection.
       Why the hype surrounding Purolator?  In my experience, just because something has been around for many decades does not mean they are the best, in fact I often find that companies like this are very hesitant to change, and they end up being not as “technologically advanced” as newer companies starting with a “fresh” perspective.  I’m not trying to slander, take cheap shots, or anything like that… I’m truly just curious.

  • Brodoc.

    If you’re talking high perfomance engines, Purolator oil filters would not be my choice. For the everyday driver basic to moderate engine these filters are fair to very good.

  • Anonymous

    Unfortunately most of the above info is in fact biased by the source which clearly has a vested interest.
    Personally I expect filtration down to 5 microns & consider 20 to be a rather pathetic figure. Certainly general practice in the hydraulics industry is 5 microns max.
    I’d like to see bigger filters also.
    The 99.9% claim from Puralator is unique & in my view somewhat suspect.

  • D-cass

    I hate to stir things but if purolator are owned by bosch the odds are that the stock bmw oil filter is exactly the same as they are most definitely bosch sourced

  • Cam

     Your so amazing. Tell us more about all the cars you’ve driven…

  • Herb

    PureONE filters are 99.9 percent% efficient – the highest efficiency ratings on the market,  Not a chance, PureOne filters are have a NOMINAL rating meaning they are only going to trap 50% of dirt at whatever micon rating is given on this filter.  They don’t mention this,  oil filters with Absolute ratings trap all the dirt at it’s given micron rating. 

    First one has to know the difference between nominal ratings and Absolute ratings.  These filters companies like PO, Mobil, K&N, R/P etc, say my filter is 99.9 efficient or 99.7 etc and are the most efficient but its a Nominal rated filter, not Absolute.  

  • Sailorgp

    I vehemently disagree with your analogy comparing a spin on oil filter to a 50′s oil bath air cleaner. I more accurate comparison would be to a pleated shop vac filer or pleated furnace filter. 

  • StockStalker1

     Every car comes with a magnetic oil drain plug…and it has nothing to do with the filter.  What filter you use doesn’t matter if you follow regular OCI’s…except Fram…they just make crap.  Something else is going to break the car long before filter selection makes a difference.

  • nleksan

    My post was not meant as a “brag” (it’s the internet, what do I have to prove?), I just wanted to convey the significant variety of vehicles on which K&N filters were used…

    I personally use Red Line 0w40 (or similar weights) in my track cars, which currently are my built and twin-screw SC’d 418rwhp 328Ci, fully built naturally aspirated ’99 M3 with a Euro S50B32 making 389.2rwhp all-motor (and weighing 2760lb), and my current project which is a ’91 E30 M3 that needed an immense amount of engine work despite being perfect in terms of the body and is now fitted with a Euro S50B32 + 6spd Manual and 3.91:1 OS Giken LSD while the S14 is being built to EVO III spec and then some (2.7L I4, 8900rpm redline, hoping for ~245 at the wheels).
    The E36 M3 has the most track miles as the E30 is a relatively new acquisition and the E46 is a dual-purpose car; just over 9000 of the 44,500 on the motor are track miles (72,100 on body).

    After seeing the engine internals during a tear-down, inspection, and some upgrading at the 7500 track mile mark, I was amazed at the shape of the motor!
    The Sunbelt Stg3 cams which have spent more time above 6krpm than below it (probably) showed no visible wear on the lobes, none! The Mahle custom pistons and Ferrea 5-angle-cut oversize valves had NO carbon buildup at all, in fact they looked almost new!
    Rod journal bearings were impeccable, and the entire oil system was incredibly clean; absolutely no buildup anywhere in the engine!

    This is with only Redline oil, changes after every track day, and AFE oil filters used as, out of curiosity, I compared them to K&N and Bosch for the same application and the AFE had 16% more pleats than the former and 22% more than the latter, and a quick oil drip test followed by a pressure test (basically air compressor blast) showed it to not only flow as well or better, it was also significantly stronger; strong enough that it would be nearly impossible to break the media by “dry starting” after a change.

    I knew from my 328 that Redline keeps engines clean, also from my day job (Psychopharmacologist so heavy background in chemistry), as being a true Group 5 oil it is ester based, and as such it can’t sludge like regular oil and furthermore it causes existing petroleum sludge to dissipate evenly throughout the circulating oil, with almost zero chance of causing any break-offs of sludge.

    Royal Purple oil, at least the SL rated stuff (i.e. real, good, and significant ZDDP add-pak), is a Group 4 and thus a true synthetic (unlike Mobile1 or the like) and while it will dissolve sludge, it does so rather slowly and less effectively. However, it does make for a phenomenal daily use oil, the additive package containing smile-inducing amounts of anti-wear additives; further, in an already-clean motor, it will maintain the cleanliness.
    Unfortunately, unless you buy it by the case and have a stock of it (trust me, it’s worth buying oil in quantity; huge amount of money saved over a season), it’ll be hard to find any of the SL stuff, and newer formulation isn’t as good an anti-wear oil as the original (although it is still excellent and better than Mobile1/Pennzoil/Castrol/etc, as well as Amsoil).

    I do UOA’s with my oil, via Blackstone and a few other labs, and my 328Ci which goes 3-5k per oil change has not only a built motor top to bottom (the actual block is the only original thing I can think of) but also running over 12lbs of boost and a static compression ratio of 11.2:1 (ESS Lysholm 2.3L twin-screw blower, modified for higher boost, air path modified to accomodate both the Individual Throttle Body conversion and the addition of a 3.9″-thick front mount intercooler which dropped IAT by over 30deg and the larger air volume actually greatly improved distribution between cylinders, from +/-35cfm to +/-4cfm across all 6 while increasing velocity 8%; benefit was a tighter tune with an extra 13 ponies on top of those given by the intercooler effect).
    Following the break-in period for the new parts, which coincided with both the new ZF 6spd MT from ’06 330Ci ZHP (19k miles on it prior; disassembled, fully cleaned, gears shot-peened and cryo treated before the entire assembly was polished; the last part had shown me an overall parasitic drivetrain power loss reduction by 2.4% in the past, and this time it dropped from an already-low 12.3% to 9.8%) and the custom OS Giken 28-plate 3.15:1 LSD break-in periods, I saved samples of the break-in fluids from each and put in new fluids (RL 0W40, 75:25 Redline D4 ATF and MTL, and the stupid expensive OS Giken LSD Fluid).
    I did a short OCI just to get any excess materials out, but let the engine go to redline (7400rpm, cut-off at 7650rpm; engine is built to withstand up to 11,000 but it’s not making power above 7500rpm to be worth the extra strain, and the whole purpose for the TS blower is that the engine is making 90% of it’s 372lb-ft@wheels max from just 1700rpm all the way through 6450rpm).
    Total was another 1500 miles on the engine and transmission fluids (I was told explicitly not to change the diff fluid until normal 15k change after break-in).

    Having done 21 UOA’s on the 328Ci since being “built”, 9 of the transmission fluid, and just recently the third diff fluid one, the results are:
    EXACTLY ZERO WEAR METALS PRESENT and that’s the sum of all UOA’s….
    On average, the RL oil has only used about 40% of it’s additive package and TBN has never been off by more than 0.2 compared to brand new RL oil…

    AFE filters combined with Redline oil for street/track or Royal Purple for street have proven themselves to me now, and being that I can get them for the same price as Bosch regular oil filters, it’s a no-brainer!