95 Pontiac Bonneville SSEi - Losing Oil

[CHEZRAY]

Have a 95 Pontiac Bonneville SSEi that is losing oil for unknown reasons.
Had a replacement engine 3.8 L turbo put in my car with about 70,000 miles on it that came from a Riviera that was rear ended. My original engine had 145,000 and needed replaced.

On the replacement engine the shop replaced all the gaskets (new), put in new bearings, plugs, catalytic converter, etc.

Got a 6 month, unlimited miles warranty. Have burned almost 4 quarts of oil in the first month (1000 miles). There are no leaks anywhere and nothing noticeable from exhaust.

Car in shop now (today) and they have ran a series of tests, compression, etc.and so far everything checks out ok. Any ideas as to where oil is going? What should I do if they can't find the problem? Should they be responsible for repairing the problem (if they find one) or replacing the engine if they can't solve problem? Thank you....chezray