Marvel Mystery Oil is very light; 55 Saybolt Seconds.
Another little point of interest. Gear oil or grease class is different. 75W90 Mobil 1 will be 450 SS. Just for reference, synthetic oils / greases are usually about half the weight of mineral oils. Example; 75W90 mineral gear oil will be 850 to 900 SS. Same with motor oils. Now you have to be careful running very light oil and mixing. Film strength will suffer. Now Jamie,,ah,,well,,ah,,I must confess, we also have a Faylax machine. You know the bearing metal to metal test for a lubricants film strength. Alabama red neck technology, I degreed the cams in a friends motorcycle and he gave it to me.
John at FHS once sent us 6 quarts of various oils, old and new tech. It took all day, 10 hours to test, viscosity, film strenght and run on the dyno. About a month later he sent us 24 quarts,,Ha Ha Ha,, and I had to decline. That was about 1998 ? when they were about to release the very first Dominator oil with the extra PTFE additive. I will say this. If we ever got back into karting we would use FHS, good stuff.
My advise: Choose an oil that is readily available, your engine builder recommends, and shows good ware inside your engines. And BTW, you don't have to change as much as you think.
Jamie, great to see your still on here, I'll try to remember to keep you informed. Now, what have YOU been holding back ? LOL
Best, WP