Perhaps a compression and especially leak down tests are in order. You don't want to exchange engines without knowing for sure that you need one. You know the oil is missing. You need some indication of where it is going. Test, don't use the SWAG method of diagnosing.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.