Synthetic (Mobile1) and change every 3 - 5k miles. Never, I don't care what the manul says, wait more than 5k miles for an oil change. Unless you don't drive your car at all. If you drive your car (keep in mind that just driving the car qualifies as hard driving) don't go over 5k.
it depends also if your driving long trips or stop and go traffic...im currently at 7k miles on my synthetic oil and its still clean...but im gonna change it soon i dont go more than 8k without an oil change.
for older cars/higher milage that were not filled with synthetic from the factory, switching to synthetic can, overtime, eat away deposits that need to stay. I use a Castrol synthetic blend and go about every 5-7k
i hear that if you switch from oragnic to synthetic you can leak a little oil. i guess that the synthetic is not as large a molecule and can go through the very small cracks that can occur. any thoughts? i am thinking of switching on my next change (this month) and wondering what to do...
I switched to synthetic from regular oil. I've not noticed any leakks and my car doesn't burn oil any more (it used to burn about .5 quart every so often). I've also heard about this possible leakage but have yet to see it on my car. It may also depend on the age/mileage of the car, but I switched at around 77,000 and now have 107,000 on the clock and no issues to report.