Wow, sounds like a nice car, especially since it was not messed with all of those years.... was it stored indoors?

I think a flush would help.... perhaps some adjustments.

If it is a nice car, I would be sure that the timing chain/tensioner/guides and rails are replaced at once if they have not already, or if you are unsure. Its my understanding that the 81' 380 has a single chain and has a higher chance of failure than the newer double chain 380 (84-85). At those miles and since it was setting, its a good chance it has not been changed, unless you have already seen to that. I would find out though.

Good luck.

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2004 Toyota Sequoia Limited 4wd
1991 Lincoln Town Car Executive
1991 Cadillac Sedan DeVille
1988 Mercedes 300SEL
1972 Chevrolet Caprice Kingswood Estate 9-passenger wagon
1973 Pontiac Grand Ville
(Prior MB's: 1974 240D, 1985 380SE, 1984 190D, 1993 400SEL)