Here's a n00bs $0.02 (not that I have no experience, just not on TBN

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Cars and pickups typically operate at 25-25% load and same for rated speed.
Trucks typically operate at 30-50% rated load and up to 100% for short periods and 85% rated speed.
Tractors? Not sure, but looking around my farming "neighborhood" the tractors are about like a truck. Lots of short use easy to moderate work and a few hours of real hard work now and again.
As long as the oil meets the appropriate specification, or exceeds it, there is no real loss of protection. The standard are minimum standards and most oils exceede them by a LARGE margin, but that margin is different from brand to brand and often within brands of different weights.
Having worked with dozens of dealers from automtive, truck and equipment I can assure everyone that in *most* cases the oil they use is BULK and not from the nameplate manufacturer. Back years ago, where I grew up Pennzoil was the predominant oil used by the automotive houses, from AMC to Mercedes all the oil was the same brand, unless it came over the parts counter at the retail level. Same for the truck and equipment dealers, just a different brand for those (Mobil).
If you use a name brand product, you don't *really* have anything to be concerend about, not at all. If the lube meets the OEM spec and most will.
To my way of thinking, telling a customer they have to use OEM fluids and filters (or implying that they really should) is an utter dis-service to the brand, dealer and most importantly the customer. The equipment is better served by frequent attention and budget fluids than infrequent attention and "premium" products from my 25 years of tearing stuff apart.