Originally Posted by MLP
For the most part for diesel engines the non synthetic oil is best because it has much better lap shear properties than synthetic oil. Synthetic oil is generally better for gasoline powered engines.
Delo 400LE 15W40 has a lap shear of 73520 psi far above the best synthetic.
Mobil Devac 1300 15W40 has a lap shear of 73300 psi
Lap shear comes into play under extreme stress. Diesel engines have much more mechanical stresses than gas engines due to the torque generated by them. So I do not use synthetic oil in diesel engines.
Shear generates heat. The more shear the more heat. So unless you have a 22 qt oil pan and a huge oil cooler, you are going to coke the conventional oil (oil temp above 270 F degrees). Synthetics do not begin to degrade until over 320 F degrees and some group 5 oil is stable to 525 F degrees (Mil Spec 7808G).
I have a Jetta 2L TDI. The only approved US oils are Mobile 1 ESP 5W30 (available at Ram dealers) and Castrol SLX gold 5W30 (available at VW dealers). Its little 118 cuin engine produces 236 ftlbs of torque @ 1,500 rpm under 26 lbs of boost, so I would say it is stressed. Oil changes (4.3 quarts) are recommended at 10,000 miles or 1 year which ever comes first.