in colorado I switched from my old standby Castrol Dexron III to synthetic. synth is better in the cold mornings but noticed no difference otherwise.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)