I have used two types. Marvel mystery oil and Synthetic 10wt. motor oil. I prefer the synthetic oil. ONe quart from the walmart will last a lifetime. YS Engines cannot use Marvel oil without damaging the o rings etc. Per the directions. synthetic does the trick for all. I think WD-40 and 3 in 1 may tend to sop up the water and not be as good a preservative as the others. Just a thought. YOur choice.