Oil leak
2 Answers
The front and rear main seals probably are leaking along with the oil pan. And possibly the oil pressure sending unit which is common to loose a lot of oil. Check on the oil pressure sending unit first, and try snugging up the oil pan. If you find the oil pan to be loose and the oil sending unit to be bad, then try adding some seal softener to a oil change to try and seal up the front and rear oil seals that seal the crankshaft.
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To have that magnitude of an oil leak with no warning (at least you didn't mention anything) .. The first thing I would check is the oil pressure sending unit because it's easy to see and cheap to fix ... anything major on a 30 year old truck may exceed the truck's value
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