My 90 350 sdl has been burning 1 qt every 2~3000 miles for over 75K miles.

Why don't you do a compression test now to set a baseline. If compression keeps dropping along w/ increase in oilo consumption, you may be ovaling a cylinder.

I've seen 3.4 motors go over 400K so I would run that motor until it actually dies.

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82 300D....went to MB heaven
90 350 SDL....excercising con rods