Small Engines (Lawn Mowers, etc.): 3.5 HP Clinton Engine, automatic transmission fluid, cycle oil

Question
I have a Moto-Mower Tiller with a 3.5 Clintonalloy engine on it. I
rebuilt the engine and it starts easy and has plenty of power.
However, when the engine gets hot and is under a heavy load it misfires.
I believe the exhaust valve is sticking. I have been told to add some
automatic transmission fluid to the gasoline. Also this engine was
built to use leaded gas. Would adding a lead substitute to the
gasoline help or would adding some two cycle oil to the gasoline help?
Any suggestions would be appreciated. I am trying not have to remove
the head to fix the sticking valve.

Answer
Hello Jim:

The 2 Cycle Oil will Work. However I Suggest you Add Marvel Mystery Oil to the Gas. If you can Purchase the Lead Additive, then Use it. It will help Stop the Valves from Sticking too. Hope this Helps. I am Here if You Require More Assistance. Let Me Know What Happens, Please. Thanks.

Good Luck

Respectfully  

John