Long considered the most basic of automotive maintenance tasks, the average oil change remains blanketed in controversy. Not just regarding what type of engine oil is best, or which engine oil additive performs on a higher level, but in how one goes about affixing oil filters. Do you pre-fill your oil filters prior to installation? Or do you just lube up that gasket and let the oil pump sort it all out?

Online opinions about this topic are all over the place , so our friend Lake Speed, Jr (a.k.a. The Motor Oil Geek) decided to put the discussion to rest once and for all by conducting some scientific research.

The mission was simple: Find out if fresh motor oil straight from the jug was dirtier than the filtered oil going through the filter and into the engine.

The methodology was equally straightforward: Send samples pulled from said clean drum of motor oil, as well as new motor oil from an engine off to SPEEDiagnostix for some ISO 4406 cleanliness tests.

Apparently, there are two ways of doing this: The right way, and the foolish way. And in a moment you will see the science that backs it all up.

Prefilling Oil Filters: Pointless Or Imperative Prep?

Lake Speed, Jr. isn’t called “The Motor Oil Geek” for nothing. The guy is a certified lubrication specialist or tribologist, and is forever the first to point out things like the importance of proper funnel storage for cleanliness, proper detergent and oil additive levels in products, etc. He also pre-fills his oil filters before conducting an oil change.

So when he heard the argument that some of his viewers were worried about the cleanliness of the oil being dumped into the oil filters before cycling, he decided to fire back with his take on the matter.

The Geek explains that by not prefilling our oil filters, we are preventing oil from performing its intended task within the bottom end of the engine upon start-up, for as we all know, internal component damage occurs when metal-on-metal friction forms. Speed claims that despite there still being some engine oil on the crankshaft, main bearings, and so forth after draining out the old liquid, it’s not enough to prevent wear from occurring during start-up.

Simply put, pre-filled oil filters prevent your engine from operating without adequate lubrication while you wait for that filter to fill.

Gettin’ All Greased Up For The Big, Oily Wrasslin’ Match

After pumping some fresh oil directly out of the drum, Speed set this sample aside so that it could later be sent to the lab for analysis. Then, the freshly filled engine was primed with an air drill,  so that the oil could hit every portion of the engine’s internals. Once that was done, he then pulled a sample for sending off to the lab for analysis.

By using an air drill and a priming driver, there should be no internal contamination of the oil due to wear, as all that is being done amounts to pre-lubing the engine.

Oil pressure was also measured, seeing how long it took for the engine to reach operating pressure with an empty oil filter, and noting that if the engine were driving that oil pump instead of an air drill,  there would be a significant amount of time the engine was operating with no oil pressure. Then, they allowed the oil to drain back into the pan, and spun up the air drill again, monitoring pressure again to see how long it took the engine to reach proper oil pressure.

The difference in the time that it took to reach optimum oil pressure/lubrication was blatantly apparent. From what we witnessed, the oil pump spun for damn near 6 seconds before the oil filter filled.

Let’s Look At Some Lab Results

After receiving the results on both samples that were sent off to SPEEDiagnostix, it was clear that the oil straight from the drum was far cleaner than the sample that had hit the oil filter.

ISO 4406 cleanliness tests focus on particulate counts, therefore any contaminants measured on a scale of anything less than 4, less than 6, and more than 14 microns in size will get flagged. The higher the test result numbers in any of these categories, the dirtier the oil.

ISO particle counts out of the drum were 16, 15, and 12, respectively. In contrast, engine samples were 19, 18, and 15, so far more contaminants across the board! Large particles that were more than 14 microns in size were easily the most astounding (and damaging), with the drum sample only having 52, and the engine sample having a count of 331.

Essentially, the oil that we put into our oil filters pre-start is always cleaner than what finally hits that filter after a few seconds of turning the engine over to the point where proper oil pressure levels can be achieved.

And then there was the topic of engine teardowns and that sort of inspection process. According to MacAskill, most bearing scratches are caused by the first 4 to 5 seconds of cranking an engine, and they only get worse when there isn’t enough lubrication present. So in the case of a first start, engines without any oil in their oil filters are spending more time with metal-to-metal contact before oil pressure is sufficient to create the required hydrodynamic wedge. That translates to more wear on your parts and more particles floating around in your engine oil while you drive.

So, the results are pretty clear. Fill up your new oil filter before you screw it on, and if possible, pre-lube your engine. Assuming you don’t like metal-to-metal contact, that is.