I have been running Shell Rotella 15w-40 in my John Deere tractors for the last 25 years. I also ran it in my '05 Ford PSD. My new '15 6.7 PSD calls for semi-synthetic 10W-30 (which would be Rotella T5) but for severe duty (by definition my tows over 1000 miles and my Texas heat qualify me for it) calls for 5W-40 which would be Rotella T6 or equivalent. My question is, on the diesel forums the guys running the T6 are claiming lower fuel economy than with T5. I assume this is because it's 40W? What about running Mobil Delvac 1™ LE 5W-30? Is it the full synthetic that I need for "severe service" or is it the heavier 40 weight viscosity?