Don't seem to have much choice in mineral oils these days. Mobil used to be good, a very dark rich colour and the 100 was thick. Buy Mobil 100 now and it's lighter in colour and seems to run quicker. Also the newer oil seems to burn more in the same engine.
Synthetic oil I have had no experience with in aircraft, in cars and bikes (Older) it seems to leak more oil when you use synthetic, same in aircraft?
The difference between a flat engine and a radial on a frosty morning was about 15 minutes in warm up time, that's 3 ton on quicker work