A little history on fuels: In addition to the things mentioned above, you need to consider the metalurgy in the engines. Old iron/steel engines would suffer "thermal runaway" if ran too lean. That is, the piston would heat and expand faster than the cylinder which would create more heat. At a certain temperature, most synthetic lubes flash off, or burn, removing their lubricating properties. When castor heats up it thickens until it eventually becomes a sticky "varnish". As the castor thickens, it slows down the engine until it eventually stops. With insufficient lube (even castor) you can ruin an engine in as little as one extremely lean run. With proper castor, it can lessen the wear. Of course, a little over lean will have less impact and you will just get less wear with castor.

ABC, ABN, etc. metalurgy usually does not suffer thermal runaway, at least not as much. The theory is that the plated brass sleeve will expand more than the high silicon aluminum piston. In this case, with an over lean run the castor will still lubricate even when the synthetic flashes.

Bottom line is that castor is insurance against ruining your engine when you have tweeked the needle a little too lean.

Also it is important to maintain sufficient lube in the fuel. Many consider 20% a minimum, and 22%-25% ideal. For some engines like the Fox .35, McCoy .35, etc. 25%-28% is ideal.

For the two engines you mentioned, the fuel is OK unless too lean, then you do not have the castor protection. Your choice.

Oh yes, you may need longer lines on that Shark 15, or you may close the carb a bit with the limit screw.

George