I wouldn't and don't use block fillers in any of the engine I build due to the oil temps and cylinder wall heat and expansion differences I'm old and I don't like a lot of the changes in a lot of the things we have to deal today
I regularly saw 15 to 20 lbs oil pressure at 850 RPM idling in gear with my duster with 180+F oil temps with that 5w20WT oil, 30+lbs at 1200 RPM
As far as oil temps my air cooled airplane motor had a oil thermostat to make the oil get up to 190 F before opening the lines to the oil cooler so it would boil the water out of the motor from condensation I used 15W40Wt in it.
I was told in commercial flight training that petroleum based oils would start to deteriorate and destruct the oil molecule at or above 325F, synthetics where good to 500+F
ATF has added dyes that will start to change colors from red at 275F and be black at 325F