IMNSHO, you will benefit by going back to dino oil of the 15W-40 variety. Given what I believe to be a "sketchy" maintenance history by the PO it is likely that this particular motor prefers the dino oil over the synthetic type of oil. Thoroughly drain whatever oil is in it, let it drip for as long as need be. Fill to specified capacity with the dino oil and see if the motor sounds happier and hopefully uses less oil. You should also address the carb/choke issue that you spoke of on page 1 of this thread. It is possible that an engine running too rich or lean could use more oil than normal. Keep us posted with the results of the change back to dino oil.
Cheers!
Mike
