The heat in oil comes from resistance mostly, so compare any Brit twin Triumph, BSA, etc......These run high oil pressures with a common shape oil tank.......The Vincent runs almost nill pressure and has an oil tank that acts like a big heatsink with plenty of air flow around it.......I read somewhere that a Vin is flat out getting an oil temp over 70 degrees Celsius on a hot day........So more of an issue with sludge forming in the base of the tank from moisture build up.........You want oils in the viscosity of 10 to 50 weight.......anything higher is too thick........I run a 15/w 40 diesel oil in mine which is still quite thin, but because of the cams in my engine it needs to spin quite fast when i kick it over or else it just wont fire up....... this no matter what kind of ignition I try, either coil or magneto made little to no difference........I don't ride a huge amount, perhaps 1000 to 1500 miles per year, so i just change the oil every 1000 or annually, the oil tank stays nice and clean, contents of the timing chest looks like new.........Good enough for me.