OK, so I went & looked at the manual.
I now have an opinion concerning oil in the air filter - use only clean new oil
- I may not use delo 100, but it will be 40wt & it sure as hell won't be used or dirty oil.
According to the GM manual -
Oil required is the same as used in the engine.
Service interval is every 2500 miles!
To service, remove cup, pour out oil & clean cup. Remove the mesh & clean it with solvent - let it dry (don't use compressed air) before reinstalling.
The dirty air must go thru the oil as it is making an abrupt change in direction. most of the dirt is slung out of the air into the oil. Some oil is carried by the air & the mesh traps this oil & as this oil collects in the mesh,it traps any fines that are still in the air as the oil drains back into the cup.
It is for this reason the mesh needs to be cleaned when the cups are serviced. My guess is that that doesn't happen often & is why paper elements are used today (- after you factor in the labor rate to do it right, the paper filters are cheaper if you're paying a shop to do it for you. )
How much dirt will dirty oil collect? How much will it let pass? How well will it drain back into the cup? How much dirt will it leave behind as it drains back into the cup?
FWIW, the original service manual for my Scenicruiser calls for 2 quarts in each cup & there are 4 cups. . . .