Dealer BS. The Mobil 1 5W-40 Turbo Diesel Truck synthetic is fantastic oil. It doesn't matter if you buy it at Wal-Mart or anyplace else.

Putting the dealership aside, what does your owner's manual say? Chances are it gives the viscosity and some letters pertaining to the ratings. As long as you use oil that meets those specs. you're covered.

And I would keep all receipts as long as you're within the warranty period.

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'81 300CD - 180K, cannot be killed by any conventional means
'99 Ford Escort - good MPG