Originally Posted by STORMINORMAN

...a 15W-40 HDEO (heavy duty engine oil) is more shear resistant than a typical 10W-40 PCO (passenger car oil) because the viscosity spread is less (there is less need for additives to extend/maintain the viscosity range).

HDEO's typically are both diesel & gas certified (i.e. CH4/SL) and are often an excellent choice for cleaning an engine (or keeping a clean engine clean). They often deal better with heat and longer oil change intervals.

For the premium price paid, typically $6-7 per gallon for dino HDEO's (Pennzoil LongLife, Delo 400, Rotella-T), these oils are usually considered superior (for older MB's at least) than typical dino passenger car 10W-40 oils (Quaker State, Valvoline, etc.) by many. There are some excellent dino oils but not as many in the 10W-40 weight (vs. 10W-30).

As indicated above, temperature range is a very important consideration. Most HDEO's are fine year-round except in very cold climates. Some 100%Synthetic HDEO's (Amsoil AME 15W-40, Rotella-T Synthetic 5W-40) are reliable down to some pretty cold temperature ranges.

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Cheers!