A few corrections/clarification related to statements in Deve's post:
GM got rid of the bypass system for a reason. It filters 15% or so of the oil and you never know which 15% it is filtering. You could have unfiltered oil from the last oil change still just sitting there. All of the gunk in the bottom of your canister is also in the bottom of your oil pan.
By-pass filter do filter all the oil after a short period of operation. Hydraulic engineers could calculate how long it would take for 99+% of the oil to have passed through the by-pass filter (I have seen charts, but am not interested in finding them). This is a fact.
All of the gunk in the bottom of your canister is also in the bottom of your oil pan. Full flow systems have much less gunk in the oil pan and all of the crankcase gases and misc dirt are filtered. 100% of it.
The gunk that settles out is predominantly due to the fact that most pre-54 (and later, I'd bet) engines used non-detergent oils. Non-detergent oils do not hold contaminants in suspension, as do detergent oils.
- Run a high-pressure 235/261 with non-detergent oil. "Gunk" will precipitate out in the pan and in the oil filter.
- Run any 216/235/261 with a by-pass filter and detergent oil. Particulates will get filtered out of the oil.
Note that GM started to suggest in 1952 that (Heavy-Duty Type) detergent oil could/should be used in heavy-duty service conditions.
