It's the Castrol power RS Power Sustain technology 20w50. It is not mentioned anywhere if it's full or semi or mineral. I can only see
SAE 20w-50; JASO MA-2 (T903:2016); API SN
EDIT:
https://www.castrol.com/nl_be/belgi...nd/castrol-power1.html#tab_power-rs-4t-20w-50
Seems to be
semi synthetic.... No good then? What do you recommend?
Yesterday I saw this in a Magazine about Yamaha Vmax
I cannot say for sure that the oil is the dominant Factor .But I use Mineral only .
The XS650 was designed at a time when these modern Oils did not Exist .. And I have over the years met many that has bought the latest " Snake Oil " Additive
and made a mistake myself with a to aggressive starter Fluid Ruining seals in a Auto Carburetor.
Jetmech Marty has a slipping clutch and I gave the advice to wait a while as Myself do --- :believe it will improve.
Does not sound so at this point in time.
If this was me in your case I would readjust the Clutch ---- but since it is hard to find neutral already .. it most likely is set tight already .
I do this having the rear wheel, in the air on center stand and trying to stop the wheel clutch in at first gear and adjust till this is OK.
If this is a Rebuild an Oil change is Coming rather soon. And then I would try another Oil
As a first step.
There is a possibility this oil was not the Best Choice. But i cant be certain
But the rationale is to use only the Fluids that existed then .And the designer then had a fair chance to test
(But the 20 W -50 is right although not exactly to spec if remember right ..but there are other considerations involved as engine wear )