back to the same old question... must we use only fully-synthetic oil in our low-compression turbo engine?! i'm currently using Mobil-1 5W/50 fully-synthetic oil and almost done 5K kms now. must i change the oil and filter every 5K kms or is it alright at 10K kms (oil filter at 5K kms of course) instead?! some says Castrol Magnetec semi-synthetic oil is sufficient for our engine and not necessary to use fully-synthetic oil. sheez, i'm getting confused now!! guys, what is best now?!