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#2: This shows how a typical transmission, in this case a drum type cam similar to a Classic K bike, might shift. #4: Some information how a transmission works; /2 transmissions, testing, details.

NOTE ALSO that the thicker oil will have more horsepower losses associated with it ...friction within the moving oil itself too ....besides the various gears, etc., changing shifting characteristics due to different slow-down and speed-up slowing.

There is a rating called Viscosity Index (VI or v.i.), which is the RATE of thickness change of the oil with temperature change.

Restating this: One difference is in spin up & spin down time for shifting. I am also concerned that, when colder than approximately 90F (or maybe somewhat more) air temperature; and/or the engine not being used at high output (which produces more heat, that DOES get to the transmission), lubrication is possibly reduced, protection could be decreased, & there are other not-so-nice things.

For example, using 80W145: The oil at any normal operating temperature will ALWAYS be thicker than if the oil was any lighter grade, including 80W90.

While there can be other causes that a change to a thicker oil will not help, in this instance you CAN try the thicker oil, such as 85W140 or similar.

My present recommendation is that, if you want to, you may use a synthetic gear oil of good quality.

I recommend that you NOT use any additive if your transmission is filled with synthetic oil.