1986 Ford Mustang

In the last week my car has started to occasionally blow lots of white smoke out the back. It stops after a few minutes. It has been burning a lot of oil and the heater core has leaked for a while, but had ceased since adding a stop leak. I drove for a an hour with the oil cap off accidently. The oil gauge appears to float around between normal and low, but no oil light. My dad said the engine is gone or the seals have given out, if so how long does it take to die? Is there anything I can do to prolong it's life since I cant rebuild? The engine is at 67000 km's (it was my grandma's car). What else can I look for myself to determine if this is the problem?