It's just that I have like 33 gallons of the stuff sitting in the basement. HE has decided to just go ahead and use 5W-20.

Which leads me to my next question: I have 3 MB diesels that don't leak or burn any oil that I could put it in:
my 190D with almost 320,000 miles now
my brother's 300SD with 181,000 miles
and our Sprinter with almost 185,000 miles now

We have ran conventional in my 190D the past 2 oil changes (10w-40 then 5w-40, 3,000 mile interval instead of the recommended 5,000 miles) but according to the PO, it was ran on synthetic Rotella.

The 300SD seems to have been ran on conventional by the PO and we have not changed the oil on it yet (haven't driven it much).

The Sprinter has been ran on synthetic Mobil 1 Turbo Diesel Truck oil (5W-40, changed at 7,500 miles instead of 15,000 miles as recommended by manual) since new. My understanding is that the Mobil 1 oil is a higher grade synthetic, but should that really even matter if we change that oil at half the specified interval?

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.