I hate to start another oil thread, but I have always heard that synthetic oil improves fuel economy. I changed my oil a few weeks ago to Castrol SynTech, and the first tankful yielded 1 additional mile per gallon (26 to 27). Now I know that one tank does not prove anything. But I check the mileage on every tank and history shows that the only reason for deviation from 26 mpg is a larger than usual amount of freeway driving, and this tank had no unusual events.

Anyway...I was wondering, if others have seen a difference in mileage with synthetic in your diesel??

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1976 240D
1987 560SL
2007 E320 Bluetec
1998 C280 (now son’s car)
1982 240D Manual - Sold

in my experience with synthetic in both diesel and gas engines, and i have used only mobile 1, i have not noticed any increase in fuel mileage with the synthetic. an 84 accord was probably my best benchmark, as i, for whatever reason, went back and forth a few times, fuel mileage using 15w - 50 mobile 1 and 20w - 50 conventional penzoil was essentially the same. some may have different experiences and perhaps lower viscosity synthetic would have an advantage over comparable viscosity conventional oil.

I use Mobil Delvac 1 which is 5w-40 syn diesel oil and have noticed a considerable improvement in economy with my daily commute. This is probally only because my dialy commute is <2mi each way (terrible on any oil and engine). I figure that the synthetic oil pumps easier than ester oil just when the engine is started and still cold (never gets to full opperating temp on my commute). Engine seems more powerful too. If my commute was longer and with highway, I'd expect to see less of a difference between ester and synthetics.

Noticed a consistent 20% (+- 5)% improvement in economy.

ATLD

I've noticed the same thing, and I seem to have to floor the throttle less to get the same acceleration. My drive to work is exactly two miles, and I instantly began to get an extra 30 miles or so per tankful after switching to the Rotella synthetic. Now I'm reading that this stuff is a bogus faux synthetic, but that's being covered in another thread. I'll probably try Mobil 1 or Delvac 1 if I can find it.

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Bob Roe
Lehigh Valley PA USA
1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic