I went to AutoZone's site and tried to price compare, but I can't seem to find gasoline-rated oil in a 4-quart jug -- all I can find is 5-quart jugs. Whereas the diesel oils are more commonly available in 4-quart jugs.

The prices I'm seeing are about the same -- meaning a 4-quart jug of Delvac is about the same price as a 5-quart jug of gasoline-rated oil.

Maybe you've got something going on locally, like close proximity to a refinery or something.

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Lots of diesel W126s in the past
Current: 1991 Toyota 4Runner with an OM603