10% or 15% Nitro is usually ideal for the .46 LA. They're not wildly sensitive to nitro content, but they can be sensitive to oil content. 18% to 20% total oil content is fine, and most sport fuels contain 18% oil. Plain bearing engines like the .46 LA will fly best with castor oil rather than synthetic oil for lubrication.

Castor oil is thicker than most synthetic oils, and castor lubrication will help "float" the crank shaft inside of the bushings for smoother performance. Castor oil can also help seal up tiny leaks and gaps in the machining of the engine resulting in better compression and more power.

Running a plain bearing engine like the .46 LA on 100% synthetic lubrication fuel won't necessarily hurt the engine, but it won't run as strongly or as smoothly as it could. As a minimum, fuel with an 80/20 synthetic/castor lubrication content would be better, and 50/50 synthetic/castor oil better yet. If you can't find 50/50 fuel and you have to run your .46 LA with either 100% synthetic or 80/20 synth/castor blend lubrication, just buy castor oil from your local hobby store or pharmacy and add about 4 ounces per gallon of fuel.

Synthetic oils are less expensive than castor oil, so most sport fuel is sold with synthetic lubricants because profit margins are better. Sig Manufacturing still offers 50/50 synth/castor and 100% castor sport fuels, but it's not often carried by many hobby stores. 100% castor lubrication would actually be ideal for most 2-stroke engines, but castor oil in large quantities can contribute to carbon buildup on the valve assemblies in 4-stroke engines. Even Saito 4-strokes can be normally operated with 80/20 synth/castor fuel, however.

A properly tuned .46 LA with 10% or 15% nitro and plenty of castor oil lubrication will run for a decade or longer and supply plenty of power. Feed it the right lubrication and work at tuning the air-bleed idle setting and high end needle as best you can, and you will love this engine!

Based on your description of rough idling and poor transition, I think you need to work at adjusting the air-bleed screw on your carburator in particular. Turning out the screw to allow more air in will lean it out. You can use the "pinch test" while your engine is idling to confirm that it's running rich at idle before attempting any adjustments.

With the engine running at idle, pinch the fuel line between the remote needle valve and the carburator shut. Ideally, the engine should speed up for a second or two then die. If the engine runs normally for a few seconds and then speeds up and dies, it's running too rich at idle. If it dies almost instantly without speeding up or while only speeding up for a split second, the idle setting is too lean.

If you turn the air-bleed idle screw all of the way open and the low end is still too rich. You can use a very small drill bit to drill out the air bleed hole in the carburator to the next bigger size. Engine gurus say this is sometimes necessary to get an air bleed carburator to idle and transition in an optimal fashion.