My two cents on Marvel Mystery Oil to help aircraft engines perform without the exhaust valve hanging open:

After 35 years of flying agriculture spray planes, piston engine types with the problem of the exhaust valve sticking open on several different types of engines, I have found that Marvel Mystery Oil added to the fuel and added to the oil helps relieve this problem. The lead accumulation on the valve stem is the cause of the valve hanging open. This Marvel Mystery Oil has certainly been a good fix for the engines I have been behind that had a habit of doing this. The overhaul shops and factory technicians should get the message and drill partway down into the valve stem guide with a little bit larger Reamer to relieve this problem by giving it room to accumulate a small amount of lead. This seemed to work in the auto industry engines. This is a very annoying problem that has caused many many dollars in lost revenue by having to bring the airplane back limping on one less cylinder and in some cases spraying off some of the load with the engine running on one less cylinder which can be very dangerous.

Sincerely,
Harold Blackledge
Just my two cents

Links:
Marvel Mystery Oil