I got a bypass filter on my 235 and a full-flow modern spin-on for my 261.
What I like about the full-flow filter setup is that 100% of the oil gets filtered BEFORE the main bearings.
A filter with bypass may be used for extra protection, like the original 261 full flow plumbing provided.
I use NAPA gold filters with 20 microns and anti-drain back. These are made by Wix !
NEVER use Fram filters! The cheap and thin paper elements inside have widely been reported to collapse very easily.
What I like about the bypass filter setup is that it traps particles as small as fractions of 1 micron AND eventually 100% of the oil gets filtered within minutes.
Also, even a clogged old filter never drops pressure, like full-flow filters without bypass can if not replaced with every oil change.
Frantz toiletpaper filters are the best. The TP element also removes a lot of water from the oil.
The best argument against using any oil filter is to avoid oil leaks and a potential fire hazard since the oil line plumbing on these engines is on the same side as the exhaust manifold !
Personally, I prefer filtering of either kind and going with metal oil lines and clean and good plumbing, inspected regularly.
That said, in the end I probably would be fine without any filter, since I never push my engines too hard, change my oil very frequently, keep the air filters clean, my valve covers and crankcase are filtered and being in the desert also forms less water inside the crankcase.
Btw: Early stovebolts had no oil filtering, because the oil at the time was a non-detergent type, where particles settled at the bottom of the pan as sludge, which required periodic cleaning and sometimes flushing of the motor with kerosene or other harmful solvents or acids.
Later detergent motor oils kept particles suspended in the oil, requiring a filter to trap them, especially when going longer between recommend oil change intervals.
