I have worked on large earthmoving equipment and semi trucks for 25 years. Have seen thousands of gallons of 15w40 pumped into and drained from diesel engines in every brand available cheap and expensive. I really can't say that I've ever seen a failure due to lubrication issues where the brand of oil was to blame. I've seen plenty of lubrication failures from going too far past the interval and I mean way past to the point that the oil had cooked into paste. Mostly I see construction equipment go 2-3 times the recommended interval because no one even pulls the dipstick much less checks the hours/miles on the oil. These are the guys that when they finally do spring for oil and a filter it's the $8 a gallon stuff from tractor supply. And I'm amazed when I see these same engines go 15-18k hours while being abused like this over and over. While I do know that there's a difference between conventional and synthetic it's really only heat resistance. But as far as conventional oils go I kinda have to say it's all the same thing. I've hauled some products for dial oil and I know for a fact that the base oil all comes from the same bulk and additives are added based on how and what name it's marketed. So basically the 300 gallon tote of shell and the 300 gallon tote of pennzoil next to it came from the same batch of oil but just different additives added to it. I like to add 1-2 gallons of Lucas to my own big diesel engines especially anything with an overhead cam. My .02
