[pdbrains]

Regular oil will break down quicker under the high heat of the turbo and if you ran it long and hard enough before changing the oil you could have caused some "damage" or excessive wear, but I never heard of damaging an engine from just using regular oil as long as the mileage was not too high before it was changed. How many miles did you run the regular oil before you changed it? As for the bad mileage it's more likely that there is something "wrong" with the spark plugs that were installed or the gap is incorrect.