The 235 had no filter, then an optional by-pass filter that, as stated, only filters a portion of the oil (the restriction is approx 1/8th inch). Funny thing is that *ALL* military trucks had by-pass filters even when they were based on some VERY civilian trucks. Some even had two (the ACKWX and the CCKW with the GM-3063 series motor).
And this was in 1940/41 when non-detergent motor was still common.
'Modern' filters are full flow, and will bypass (not filter any oil) when the flow causes a pressure differential... generally above 4000RPMS or whatever the spring design is. But generally about 50-80PSI. So in a working system, the filter will *never* block the flow of oil... a relief valve opens and bypasses the entire filter element. I'm sure there were all sorts of different filtering methods attempted over the years.... but issues with a working filter are rare.
The proper way (via the TMs I have) are to change the oil when the engine is HOT... like within 10 minutes after shutdown. The filter is changed when the element is visibly dirty. And that also included draining the filter canister... guaranteeing oil all over the engine compartment/engine.
