[JerryH]

I don't recommend any specific brand of oil in an oil thread, because they are pretty much all the same. I use 20w50 non synthetic car oil, and change it as often as possible. The more wear an engine has (which usually goes along with age) the less ring seal it will have, and more combustion gases will find their way into the crankcase, which means the oil will get contaminated (turn dark) faster than in a newer engine. I change the oil in my '85 every 1,000 miles or 3 months, whichever comes first. Right now 3 months comes first, because of ongoing mechanical issues. The last thing I want in my engine is dirty oil, because I'm fairly certain the actual engine itself is is good condition.

 

[JerryH]

Older engines, especially four cylinders, are not good candidates for synthetic oil. They are likely to have worn tolerances, and worn seals, which may still hold conventional oil, but synthetic, with it's smaller molecules, will find it's way through. And as an engine becomes more worn, it needs "thicker" oil to help take up the space between the looser clearances, such as con rod and crank bearings and ring to bore. Heavier oil has more load bearing ability and shear strength.

 

[JerryH]

I saw a video on Youtube where someone drained the oil out of a Honda Cub, filled it with used (and strained) french fry oil from a fast food place, and rode it several thousand miles. It was still running fine.