One quart every 15-20 miles!! If it was the rings, I don't think it would even start. I would look for oil getting pumped into the intake (turbo seals). A completely blown valve quide or other internal engine cause would give you some SERIOUS running issues that I think you'de notice. I'd look for a major external pressure leak also (oil filter lines). What color/ quantity is the exhaust smoke (have someone drive and follow them). Fill only to level half way between "add" and "full" on dipstick. Get the tests engatwork suggested.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)