Motor oil works fine for fogging — and the cat is safe

Yes, you can use regular motor oil to coat the cylinder walls, and it won’t hurt your catalytic converter if you do it right.

The method is simple: pull the spark plug, squirt about a teaspoon of oil into the cylinder, pull the cord a few times to spread it around, then put the plug back. That’s honestly all fogging spray is — aerosolized oil in a can. Motor oil does the same job for a fraction of the price.

As for the catalytic converter: it only sees engine exhaust when the engine is running. If you’re coating the cylinder and then parking the saw for the season, the engine isn’t running, so no oil ever reaches the cat. And when you fire it up in the spring, that teaspoon burns off in the first minute. That’s nowhere near enough residue to clog or poison anything.

Also worth knowing: the “spray into the carb while it’s running” method that fogging spray is designed for actually sends oil smoke straight through the exhaust and over the cat — and that’s an approved practice too. So you’re not taking a risk either way.

Don’t overthink it. A teaspoon of motor oil is a lot cheaper than a scored cylinder.