Nice car! I have a pretty huge stack of records for the 300D because my dad did a good job of saving them. They only go back to 1995 though ... not sure where the records before that are, but I know it was serviced at the dealership (mostly) until 2002. The last dealership service invoice I have recommends the car be "retired." Everything I do now, I keep in an Excel log. It's not the fanciest thing but it's easy for me to follow: Date, mileage, items or fluid replaced, brand of replacement parts and where I got them from. I actually keep a separate one for just oil changes on the 300D (not sure why; I guess because in the beginning it was the only thing I did myself). I also have one on for my 240D, which came with zero records. I recently started one for my Jeep but I've been pretty bad about remembering to enter data, probably because I work on it fairly infrequently.
Now, I had a really cool fuel economy Excel sheet for the 300D ... but I haven't updated it in about 30,000 miles. I HAVE kept almost every receipt from every fill up with the mileage written on it. So someday I'll actually go in and enter all those receipts. Haha.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)