The oil is lubrication, & provides cooling for the stator windings and the starting clutch behind the alternator rotor. That's why it's important to use the correct engine oil, not some oil designed for a car, as "Oil is Oil" does not apply.
JASO OIL SPECS:
Find an oil that shows the JASO-MA rating on the bottle; there are plenty of them available both dino and synthetic.
The ingredients in oils have changed over the years. For example, today's Castrol GTX oil (and pretty much all other AUTOMOBILE oil) is not the same as it was in the 1980's. Back then it worked great in KZ bikes. But, things have changed. Early AUTOMOBILE oils contained zinc phosphorus compounds (ZDDP) to protect engines from metal to metal contact. This was good for engines, but it was found to be destructive to catalytic converters. Consequently, in AUTOMOBILE oils ZDDP was virtually eliminated and replaced with different chemistry generically referred to as "Friction Modifiers."
These work fine and offer good protection for normal AUTOMOBILE engines. However, today's AUTOMOBILE oil "Friction Modifiers" are destructive to wet clutches. KZ bikes (and many other brands) have wet clutches. As a result of these oil chemistry changes a distinction needed to be made between those oils which are suitable for wet clutches and those which are not.
That distinction was documented by the Japanese Engine Oil Standards Implementation Panel (JASO). JASO developed a number of different oil specifications for different applications. The specifications that apply to 4 stroke motorcycles fall under the "4T Specifications." Within the 4T specifications the JASO MA class oils are specifically formulated for 4 stroke motorcycles with wet clutches.
Using oils that are not designated as meeting the JASO-MA standard may be destructive to KZ clutches. Here's a link that may be helpful:
www.oilspecifications.org/jaso.php
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