The way we're doing it in my bro's 300SD is an initial change then a couple changes after that. You can get bulk ATF from Sams for pretty cheap, 12 quarts for $23.48. It's some off brand but I figure it's fine if it says it meets Ford/GM standards and on those old transmissions Mercedes just specifies ATF.

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.