the point behind the rotella is the zinc for the most part. Personally, i don't think its necessary on the c10, or any bike TBH, and run whatever MC oil as well, but the point was what's comparable to t6 (if you cant get it).
VR1 ...its off the shelf 9.75 a qt, 10.75 for synthetic version here @ the parts store. im sure its cheaper on amazon or whatever. I run 7 qts in my big block ford and it holds up really well, and im not easy on it at all. reason again being, big flat tappet cam.
I -have- run it in a gearbox on an old 2-stroke (my t500) and its fine on wet clutches and the gearbox seems to like it. I run castrol synthetic mc oil in the C10.
here you go. 6 bucks a qt at wally world.
https://www.walmart.com/ip/Valvoline-VR1-Racing-10W-30-Conventional-Motor-Oil-1-QT/196957962