To me the Delvac I is the best all around oil. Delvac I can be used in my 2 diesel trucks, 2 grocery getters and 2 lawn mowers. Also I like the 5W40 grade the Delvac come in.
Which is the best quality, both are quality oils. I find the versitality of the Delvac I would suit my needs better.
If you rule out large commerical diesels then Redline is Top Box!!! It is the most shear stable and temp stable oil you can buy. SOme large diesels engines builders do not want friction modifers in the oil. Delvac one does not have any friction modifers. Redline makes it's product in 5W20,5W30,5W40,10W30,15W40, 20W50 and straigt weights 5,10,20,30,30,50,60,70 as well as comopressor oil, shock oil,p.s..........THe 5W40 and 15W40 are specificcal made for diesel engines but not fleet type oils!
I doubt that most people would ever be able to tell the difference between the two products in actual daily useage. One point to remember is that Delvac 1 is an API rated oil so if you are concerned with warranty requirments that might be a factor. I prefer Redline products and do have a slight biasis but their products are not API compliant and that must be factored in!
I'm glad you're not factoring in Schaeffers and also included the bias statement as I'd have to say that your statement is unfounded if it was. Redline is a good oil but I've yet to see any good extended oil drains with this product particularly in a diesel engine and I don't think I've seen any wear #'s provided by redline to show that the extra price they get provides much if any better protection. Also, not being api, makes it less appealing for my use in any new equipment.