After the engine transplant, I put in the STP brand 10-30 oil and a new filter. It is the Autozone "House brand" oil now and relatively cheap.
SAE 30 may be a bit light. The ZZR/ZX 1100 manual specifies a minimum of 40 and a maximum
of 50. 10W-40, 10W-50, 20W-40 and 20W-50 are listed as suitable. As 15W-40 and 15W-50
fall within the required range, they should also be suitable.
I can't find much information about the STP 10W-30. The packaging says it contains
"anti wear additives". As STP are one of the biggest suppliers of friction reducing
oil additives, it is quite likely they use such additives in their own brand of oil.
Perhaps you have found one of those rare cases where using car oil actually
does cause clutch slip :)
So my plan is to change the oil and filter soon....
I would do that sooner rather than later. A slipping clutch will cook the
plates fairly quickly. Remove both drain plugs when draining the oil.
...and use the Rotella T4 and see what happens.
15W-40 formulated for Diesels. Should be fine in the bike.
Rotella T6 5W-40 synthetic is specified as meeting JASO MA or MA2
standards for compatibility with wet clutches. Unfortunately, it is not
available here in Europe, but can be bought for a reasonable price on
your side of the pond.