There are only two ways to check transmission oil on your YSR. Drain and measure, or use the Genuine Yamaha Dip Stick.
In the drawing #1 is the dip stick, #2 is the maximum level #3 is the minimum
TCR Recommends Castrol GTX 20-50W motor oil. (Remember this is a wet two plate clutch which does not like synthetic oil) Drain oil, measure 600cc (20 ounces) and pour into transmission filler.
If your YSR is leaking oil refer to this page: https://www.teamcalamari.com/dox/tekoil.html