Four-stroke engines tend to run a much lower temperatures than do the two-strokers. This means that you can use synthetic oil quite well. With rich running, synthetic oil will greatly reduce the varnish and carbon buildup you'd see in two-stroke engines.

Also because of the lower operating temperatures, you can get away with a bit less oil. With a few exceptions, all of the O.S. Max four-stroke engines can be run on fuels with the oil content as low as 16%. With hotter-running engines, the oil is used to help carry away the engine's heat. Synthetics will do this nicely until the engine gets too hot, where it vaporizes and, PRESTO!, no oil. Castor will start to turn to varnish, but will still protect the engine somewhat.

We have no problems with anyone using a fuel with all-synthetic oil in a four-stroke engine. The main thing to consider is that most fuels with synthetic oils do not provide the corrosion protection the lower-end of your engine needs. This means that you have to make sure you get a LOT of a good-quality after-run product into the lower-end of the engine at the end of the flying day. You'd be surprised to see how fast the bearings and crank can corrode if you don't.

For convenience, though, we'd suggest that you use the same 18% oil-content fuel in your four-stroke engines as you do in your two-strokes. If you use a synthetic-castor blend in your 2-strokes, then it won't hurt your four-strokes.

Good-quality fuel is the main requirement. If you use a good-quality fuel, you should have no engine problems caused by the fuel.