I'm rebuilding a 1976 GL1000. I'd like to use a synthetic oil, but I don't know if it's OK to use it in an engine this old. I don't think synthetic was even available back then when this bike was new. Any issues with the clutch slipping? There are apparently 50,000 miles on the engine. Could be more, I suspect the odometer has been tinkered with. Anyone have any thoughts about the oil?