I am doing my own Fuji 80 projects here, so I've been looking in to it. The book says 30wt non detergent oil.
The reason they stipulate non detergent is that this oil allows particles to settle into the sump. Same reason Briggs and Stratton used to specify it. Detergent oil keeps particles in suspension, so they can be filtered out. The non detergent oil is becoming harder to find, if not impossible in some places. Tractor Supply had it in my town.
You won't find a ton of knowledge or information out there on these engines. Something I have read on Hodaka forums however is that it's permissible to use the current 75wt gear oils designed for two strokes with wet clutches. Since I am now on my eighth attempt at removing the clutch to figure out why it wont disengage, I have drained the oil that many times. Once I figure out this odd phonom, I will replace it with 1.5 pints of modern 75w two stroke lube.
I mixed at 20:1 and it's running great with 91 octane no alcohol gas. It starts cold by about half a kick with a flip flop. Best running two stroke I've owned. Just doesn't clutch worth a damn.
Note that engine was used on a lot more than just Rupps. I have a couple of manuals coming in, and I will be building at least another engine, so if I learn enough to garner proper torque for the clutch nut, I'll be an expert.