I always ran 4oz per gallon on a Rotax although I think the manual even recommends less.
You shouldn’t mix different synthetic oils with each other but you can mix castor with synthetic. The thing is you don’t know what is in the other fuel so don’t mix anything with it if it’s going to sit over night. Use the old stuff until your tank gets low then use the new stuff. You don’t have to purge your system or do anything special except check your jetting as mentioned above.
Keep one bottle marked “premix only” that way three weeks from now when you’re scratching your head wondering if there’s oil in that (clear looking) gas, you’ll know for sure by the bottle it’s in.
FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience