Even more important is the High-Shear High-Temperature MINIMUM specification in SAE J300. In tables below you will notice that there are "two" SAE 40 specifications, one with minimum HSHT value of 2.9 cP for Automotive Oils (SAE 0W-40; 5W-40; 10W-40) and the other for Heavy Duty Oils (HDO) (SAE 15W-40; 20W-40; 25W-40; 40).
This double specification is at insistence of heavy duty engine manufacturers who have required HSHT viscosity limits consistent with good engine durability in high-load, severe service operation. HSHT value of 3.7 cP or 27% more viscous oil at 150ºC (300ºF).
Yes, a 27% increase in viscosity makes a difference between Automotive engine that lasts 100,000 miles and Truck engine that lasts 1,000,000 miles!
When you consider that most Automotive Motor Oils are ONLY 3 cP, while our SAE 5W-50 SynLube™ Lube-4-Life™ Motor Oil has rating of 5 cP, you can readily appreciate why we can claim 300% to 500% increase in typical Automotive engine durability, and that is with substantial "safety" reserve!