Without oil sample testing on the specific vehicle involved.
[Remember,Manufacturers lengthen intervals to make maintenance costs SEEM
lower...just as they RECOMMEND lower viscosity oils to try and meet MPG goals]
What kind of oil are y'all using synthetic OR DINO?
Without the sample testing you will never know the ongoing effects.

Who is your dealer? Always nice to know the Ethical Service Centers.

The other concern is that Mobil has started using group 3 base-stocks.

(We are running a Ford six cylinder 24 valve on Mobil1 5W30 at 5K oil+filter changes
['ran the first 5,000miles on the Ford initial fill product].At 105,000 miles presently.
It's using 16 ounces of oil per 5,000 miles just like clockwork.)

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'84 300SD sold
124.128