Dear Marco, the oils you refer to are for gearbox use only. Or rather, 75W-90 is a gearbox oil only, 15W-50 can be used both for the gearbox (in 2-strokes) but also in engines, but only on 4-strokes. 2T oils not only have the task of lubricating the cylinder/crankshaft, but also of burning together with the petrol during the explosion, which is why it makes no sense to go with overly complicated formulations, i.e. the multigrade oils in question.
More simply: when the first ones burn, they give off the characteristic “perfume” of 2T; the latter, if it happened, would make the typical foul-smelling white/bluish cloud of when a 4-stroke engine breaks down.
However, the reasons why these different types of gearbox/clutch oil are used are the need for a thicker lubricating film on the Aprilia RX 125, due to greater tolerances between the gearbox/clutch organs and the guarantee of performance in a range of higher temperatures, guaranteed by the use of a specific oil such as 75W-90, the result of a precise design choice to reduce wear precisely in the meshing phase. Instead, using 15W-50 makes life easiergiven that it is a common 4-stroke engine oil that can also be used in 2-stroke gearboxes.
In these acronyms, introduced in 1911 by SAE (Society of Automotive Engineers) to classify oils based on viscosity, the first number refers to cold viscosity, for winter use (symbol W = winter). The second number considers the hot one (not due to the climate, but rather to the engine operating temperatures). The latter, therefore, is the viscosity index of the oil at elevated temperatures. The higher it is, the better the viscosity of the oil will be maintained at high temperatures.
