It has been a while since we published the first five parts our "Lubrication for SHOs Study." Things have changed, and thanks to reader feedback an update is now in order.

First, AC Delco has discontinued the top shelf AC-UltraGuard line. They can't be found except in the remaining private stock of those folks who purchased early, purchased a bunch and hid them well. This is a singular loss because the filter cleaned down to 8 micron size even at very high flow rates. Bummer.

Martin Eble made the helpful observation that substituting a larger oil filter may change the bypass characteristics because the specifications on the by-pass valves may be different. Actually the valve specifications are identical, but the larger filter will have less resistance at the same flow rate and therefore it will open at a higher rpm.

As an example say the small oil filter starts to by pass at 2000 rpm. If a larger filter can delay that opening to say 2500 rpm that means that means an V8 owner in OD can delay opening the by pass valve from 55 mph to 69 mph. Think of a long trip, it would better to filter all the oil the whole time than by-pass much of it. You will arrive with much cleaner oil without slowing down. At say 4000+ rpm both filters will bypass most of the oil so any size advantage disappears.

But the choice to run the big filter in the stock location is not a simple one. A larger filter is about a ½" closer to the exhaust pipe in V8 SHOs and you risk baking your oil. Heat is the enemy of oil. How real is the problem? I don’t know, Paul Nimz made a nifty heat shield for his oil filter. He got a sheet of perforated metal and mounted it to the oil filter with clamps using non-conductive little blocks to keep the heat from transferring to the filter. A nifty mod for any size oil filter.

Mobil 1 changed their formulation at about the same time I first tried the large 1 quart filters. Initially the oil analysis indicated a large step increase in % oxidation and it was not clear if this was due to the change in oil formulation of filter size.  Subsequent oil analysis showed it to be a function of the new oil analysis and not of filter size. To make this clear no significant change in analysis, be it % ox or wear metals can be attributed to filter size for better or worse using either size filter.

In this test Mobil 1 5w-30 was changed with filter every 5000 miles on a V8 SHO. Heat was a concern because of the proximity of the filter to a exhaust down pipe.  The significant concern remains that using the larger filter on engine that calls for the smaller one will void factory warranty. Using the smaller filter for an engine that calls for the larger filter is never recommended.

The big issue - at least for V8 owners - is that the wrong size oil filter will void any warranties. Say Ford in their immense charity does a cam recall or compensates those with welded or pinned cams. - No small filter, no bucks. Say you have the last V8 SHO on ESP, cam goes and you have a big filter. Would Ford use that to deny a $10,000 warranty claim? In a 1/10,000 of a NY second. Fact is don’t ever let a dealership service department see your car with the wrong size oil filter. Suffice it say if you choose to run an "unauthorized" big oil filter don’t document it well and resolve to make it a secret you take to your grave.

One other thing I learned is the media in a Mobil 1 filter only looks the same as a K&N oil filter media. The Mobil 1 filter has much higher synthetic fiber content designed to filter down to 10 micron while the K&N is designed for "more sporting" applications and filters down to only 20 microns. The Mobil 1 is focused on deep cleaning while the K&N is optimized for higher flow.

Say you have a small block Chevy, all bearing clearances are at least 60 microns, so you would need (3) 20 micron to line up just perfectly to bridge across the lubrication layer and scratch a bearing surface. Unless you use that engine in truck pulls or off road that should never happen. But the bearing clearances in a V6 SHO are all about 40 microns and V8 main bearings are 20 micron, V8 cam bearings are 40 micron. So bypassing 20-micron size particles is a whole lot less attractive with a V8 SHO than either a V6 SHO or small block Chevy. In theory one could run the K&N when they know they will be running higher rpm because the lower pressure drop across the media will in effect delay by-pass opening and improve the quantity of oil. It may work for the V6, but it would be a risk with the V8, so I can no longer recommend the K&N oil filter for year round use with the V8 SHO motor - save it for track use when full flow is critical.

One last pearl from my research: one compelling argument for the big one-quart filter is its dirt capacity. My 4x4 used to have a 12" diameter by 6" tall truck filter. That filter will hold a whole lot of dirt before airflow was effected. In the same way a 1-quart filter with twice the media should hold about twice the dirt as a filter half that size. Fact is - exception being off road events like Pikes Peak hill climb, some of which is not paved - the small oil filter will never be near capacity for normal street-highway use and recommended change intervals. If the small filter has ¼ dirt load when changed and the big filter has 1/8 load what will the 7/8 capacity not used benefit an owner? If you do a lot of driving in dusty areas the big filter may be best for a very few owners, otherwise the small filter is preferable.

The last man standing, at this point, is the Mobil 1, we shall morn the loss or the UltraGuard filters.

Special thanks to Champion Labs, makers of the K&N, Mobil 1 and many other branded filters for their candor, wisdom and generosity of spirit and time which made this update possible.

Happy Motoring,

Buford