This is wild—my '88 consistently gets over 20 mpg.
In my experience, it’s rarely a coincidence when something changes. It almost always traces back to the last thing that was altered or disturbed.
Something worth checking: I recently learned about a potential vacuum leak source. The thermoswitch on the back of the throttle body controls the ambient air valve that draws "warm air" from the oil tank into the throttle to prevent icing in cold temps. If this system fails, it can also lead to increased oil consumption.
I plan to eliminate that system entirely. It might be worth ruling it out on your end too—though I doubt it would account for such a drastic drop in fuel economy.
http://forums.pelicanparts.com/porsche-911-technical-forum/987569-ambient-air-valve.html