I second the MMO. It is old school stuff that works. I had an excessive crankcase pressure problem on a 940 T. I couldn't figure out what was causing it. I finally put about an ounce of MMO in each cylinder and let it sit all night. I also put a small amount in the oil and drove it maybe 30 miles and then changed the oil. The difference was night and day! No more excess crank pressure, and it now runs like a scalded dog! The compression was blowing by the rings, into the crankcase causing the high crank pressure. If additives don't work, then you may have to do a valve adjustment as Jim said. Also! Run MMO in the gas tank. It will keep the top end lubed, along with lubricating the fuel pumps and keeps carbon from building up in the combustion chamber.

I know this as I had pinging, or detonation on my 245 wagon, with the engine under a load (like too high a gear on an uphill) After a few tanks of gas, I noticed no more pinging!