Currently I use synthetic Rotella in my Benz and Mobil1 TDT in my TDI. I could only find M1 in quart bottles and they were $2 more per quart than the gallon jugs of Rotella. The performance difference between a mostly group 3 oil like Rotella and a group 4/5 like M1 is supposedly minor and to an older diesel I don't think it makes any difference at all.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL