Hi Christian, Buzz here. Back to one of your questions, the difference between turbine engine oil, Recip engine oil, and auto engine oil:
Turbine oil has been fairly well discussed, it's synthetic and made to run at much higher tempertures than Recip oil.
The oil we use in reciprocating engines varies a bit, one difference is if you operate a big radial engine you generally buy oil by the barrel: a DC-3 has 29 gallon oil tanks, we fly with about 25 gallons aboard, and we plan on an oil burn / drip / loss of about 1 or 2 gallons of oil per hour per engine. Other engines are higher... the B-29 has 55 gallon oil tanks for the R-3350 engine. As MD11 Engineer pointed out, "products of combustion" : carbon, avgas, water seep into the oil supply so you change oil often.
Recip oil comes in straight weight: SAE50, SAE60. Multi viscosity is becoming widely available now, i use 15-50 Aeroshell in the taildraggers i fly around for fun. The Aeronca Champs and Piper Cubs have "flat" engines. Phillips makes an oil for Radial engines, it's also multi-vis.
In WW2 the engine oil would accumulate sludge in the nooks and crannies, it was often called "Mineral" oil and had few additives. In the '50's "Detergent" oil came out which causes the sludge to circulate through the engine, many people back then figured it was a bad thing to use it in an old engine.
Now there are "ashless-dipsersant" oils that don't clean the engine, but keep the microscopic bits in suspension until you drain oil. And the additives don' t leave much of a residue (the ashless part) if they get burned: leaky rings.
Auto oil has different additives to improve wear compared to Recip oil, many auto oils have a phosphorous compund which works great. Many large radials have silver plated main bearings, and the phosphorous destroys the silver, so that's one reason to follow the manufaturer's list of recommended oils.
When the original engine builder doesn't support your enigine anymore (Pratt + Whitney lost interest in their radial engines) then we ask the rebulder, Precision Airmotive near Seattle is a 2nd source.
Buzz Fuselsausage: Line Mechanic by night, DC-3 Crew Chief by choice, taildragger pilot for fun.