Join Date: Mar 2015
Location: Adelaide, SA, Australia
Yep rat540 has some very good info.
I wouldn't go to low viscosity though, remember on a motorcycle the oil lubricates the gearbox too.
Gearboxes typically rely on splash feed lubrication rather than pressure feed. A thin oil can drip/fling off to easily. It's a balancing act between different needs. The manufacturers specs are best ,taken with your ambient temperature range.
But yes , you should go slightly thinner rather than thicker for improved oil flow and lower oil temperatures.
Keep in mind also that as a motor wears oil clearances increase and that the gearbox tends to chop up the oil molecule strands causing it to thin out during use.
If you cant fix it with a hammer, its probably electrical.