Maybe a dumb question (not the first) but can dry oil have a ph? I presume you are concerned about corrosion, but as long as water isn't introduced, is it really an issue? My limited understanding is that to be acid/alkaline, you need to be in solution with water and I can certainly understand how mixing water with used motor oil could raise or lower the ph of the water due to contaminants in the oil. But...if the oil's dry and you don't add any water to it, it's not corrosive is it???
Technically, no. Oil with no polar (water-like) solvent in it can't have a pH, because there's no way to have excess hydronium in solution. However, in the real world, you have to assume that there's something polar around that can cause you grief, like water in the bottom of the drum. The chemistry way to check for acid/base in a nonpolar solvent (oil, in this case) is to shake up an oil sample/water mix in a seperatory funnel, let it sit and settle (like vinegar/oil salad dressing) , and then draw the water fraction off the bottom for pH testing. The dishwashing detergent bottle left to settle upside down is a great way to do this on the cheap.
You'll want to test some new motor oil to develop a standard procedure. New oil should test somewhat alkaline because of the buffers added during manufacture. If the WMO was changed promptly, it should be alkaline as well. Oil that was not changed in a timely manner goes acidic (especially in a gasoline engine that ran cold). Acidic oil combined with water vapor made during the combustion process in the engine results in an acid bath for internal engine parts...which is why we change the oil in the first place.
If I had some oil that tested alkaline and some other oil that tested acid, the first thing I'd do would be to mix the two, heat, and stir a while. The remaining pH buffer in the alkaline oil might just solve the acidity problem...
Diluting before filtering (centrifugal or exclusion) will definitely help with throughput. It'll also increase the extraction efficiency of a centrifugal filter.