Mobil Delvac 1 ESP 5W-40

I use a 10w30 syn blend in my 12.7 60. 530,000 miles on engine, after the factory reman. I think the engine runs much smoother with it than 15w40. Have used it for 3 oil changes and used oil samples are as good or better than any 15w40 samples. Only use about 2-3 quarts in 20,000 miles on average, and I have gotten a slight up tick in average mpg. Not enough to shout from the mountain top about it, but mpg is better. As for a 5w40 full syn, I could get that for about $22 a gallon delivered free to my front door (along with free oil sample kits and analysis) from my oil supplier if I wanted to use a 5w40. The 10w30 syn blend I get, about $18 a gallon, delivered free, and free oil sample kits as well.

On this report, compare the first two oil samples with 10w30 and second two with 15w40.