My 340 is blowing blue smoke

Read the plugs,, if oil accumulation is on one side of the electrode,, it is likely a valve guide seal, if oil is evenly distributed around the plug,, - rings..

If you back-off down a long hill, then give it some gas,, and get a blue cloud,, Valve seals/intake gskt..

If you're on it hard,, and smoke's curling out the exhaust.. rings..