Since 2-cycle engines are less common than 4-cycle engines, 2-cycle oil is consequently less plentiful. You can still find it online or in automotive stores like O’Reilly Auto Parts, but you might have to look harder or pay a little more money. Due to this, you’re not very likely to accidentally use it in the wrong kind of engine. That said, what would happen if you did use the oil intended for one kind of engine in the other, and vice-versa?
It has been suggested that 2-cycle oil is safe to use in a 4-cycle engine, albeit sparingly, since the oil in a 4-cycle engine is only meant to clean and lubricate, and the 2-cycle oil should be able to handle that without notable risks. However, it certainly won’t be as efficient at cleaning, and the vast majority of advice is to not try to substitute it at all.
On the flip side, 4-cycle oil should not be used in a 2-cycle engine. 4-cycle oil is not engineered to mix with fuel, since that’s not something it would normally be doing, so when a 2-cycle engine does try to mix them, there’s going to be some major smoking and wear.
