There shouldn't be a noticeable difference there. Try adding some more oil to see if you get better ring seal. I doubt it but it's easy to try. I think you've got something else going on. Check your spark plug and clean your air filter.

Steve, what's the model number of your Lawn Boy.

Model 7268 I was going to add that it's 30 years old and that could be one of the problems. Just to let you know I drained the gas tank and put fresh gas (not that the other gas was old) and new LB oil seems to be running right. It stopped smoking like it was doing on the last few cuts.This is the best peice of machinery I've ever owned.

Steve: As I have stated before Synthetic Oils are Great in Newer Engines . Your 30 Yr Old LB was experiencing blow by causing th smoking and diminished RPM due to Plug Fowling . If you had pulled the plug you would have seen evidence of the fouling . I have had the same experience with Older engines when Syn was introduced. If you have used LB Oil for 30 Yrs , stick with it , it's proven itself in your unit .