Just a thought if it the synthetic it probable actually lighter then the Harley formula+ stuff and if it GL5 rated you probably should not use it. Its what I put in mine but after some research I plan on switching back to the Harley + since it so hard to get a gearoil without it saying gl4 or 5 and its for differentials with out brass. Mobil says to use their 15-50 motor oil over the GL5 EP 75-90. I am not sure I would use motor oil. However do go too thick because the transmission has some small needle roller bearings inside the primary drive gear that the inpute clutch shaft comes back thru and too thick here is not good.Just my opinion.