The warning “Do not mix with other additives” is intended to prevent the simultaneous use of multiple additives in engine oil. This is because different components may react chemically and impair performance.
On the other hand, gasoline additives (fuel additives) act on a different system than oil, so there is no problem with using them together. For example, using a fuel additive like Shuraster's Loop Power Shot with Bell Scooter can provide benefits for both the combustion system and the lubrication system.
You can use Bell Scooter safely with the engine oil currently in use (Yamalube Blue SC 10W-40 MB). Please use it in the appropriate amount (5–10%).