the os 40 was it an la or fx. the la has bushings and therefore will not rust out its non existent bearings that an fx would have. I use air tool oil at 5 dollars a a quart it is very economical. I run my engines out of fuel at the end of the days flying to remove any alcohol and then put a couple of drops of air tool oil in it. Mostly I do this because I have 8 aircraft and any plane I fly one week might not see action for a month or so. I also cap my carbs and plug my exhausts. the exhaust mainly to keep residue from running out and ruining the inside of vehicles. I regards to synthetic oil it has the ability to allow engines to be burned up easier then castor oil when the engine is lean out to a dangerous setting because synthetic breaks down with heat easier then castor but you still have a destructive situation with the castor just the damage accumulation is much slower with the castor because it breaks down slower but also leaves a plane slimier then synthetic.