all synthetic is good for 4 strokes becaue of the reduced heat, but synthetic oils have a habbit of decomposing at high tempuratures. for a 46 fx, you want either a mix of the two, or pure castor. the problem with pure castor is you will find that after time, there will be a soft varnish building up on the top of your piston. a lot of rc enthusiests consider caster oil a nuissance because that soft varnish needs to be removed from time to time. i would use a blend of the two if i were you.