Well its 5W-40 not 15W-40. 15W-50 is really too thick for 99+% of applications. I believe 5W-40 is reasonably close to Delvac1 with a bit less zinc. There are various threads on this subject. It really a matter of deciding whether your vehicle needs the heavier viscosity than say a 10W-30 or 5W-30. Reasons could be extremely hot temps and or vehicles which may do some heavy towing or engines with looser clearances.
Good answer AL. It is 5/40 to be used where 15/40 is called for. I read some threads about trucks and the fleet using Delvac and Delo but this statement I found still baffels me.I believe it is almost impossibe to measure fuel economy variances - mineral v synthetic - due to the multitude of variables involved. And I monitor every truck and driver for fuel efficiency.We search for a modern tech blend and the owners still use the Petroleum base oil.You would think that might have changed by now.
Well your right AL if I looked I can find a lot about this but this hits the nail on the head for those searching for oil. My Vortec started making start-up noises once the temp dipped below 0 F. At the time, I still had the dino 5W-30 for break-in. I put in 0W-30 and no more noise, even at -42.
However, in summer hot temps, especially towing +8,000 lbs, I get a LOT of ticktickticktick noises unless I run M1 10W-30. I tried M1 15W-50 in April of 2001 before heavy towing down to Utah and Nevada and loved this oil: motor VERY quiet, no oil consumption at all.
When you consider that the API/SAE "Starburst" SL oil is allowed to thicken +275% over 96 hours and still pass the test with flying colors, why worry about running a stable oil like Delvac 1 5W-40 that is purposely designed NOT to thicken?
This whole issue of folks getting outraged over running a stable 5W-40 or 15W-50 oil is really something. Why not run the thicker, more stable oil? Since I want to keep this truck +10 years, I'm for anything to extend its life.
As M1 Truck & SUV 5W-40 IS Delvac 1, both have a TBN of 12. SHould prove to be a very stable oil, especially for hard use in high temps. It does exceed CI-4, which requires an oil to avoid thickening with severe soot loads.