I use about a quart of motor oil every three to four gas tank fill ups for my 1973 142. Gas mileage is okay-- about 15 city/20 highway. I got the car last November and I've put on about 5,000 miles since (and there has not been a single day when I have not driven it since). I've gone thru a dozen quarts of oil already (I've bought 4 qts each visit to the Autoparts store and I just poured in the last quart of oil after my third visit), and I'm beginning to be get the nagging feeling that something major needs to be done.

Is this a sure sign of worn piston rings? Or are there other possible causes?

Never once has there been any visible smoke coming out from the exhaust pipe. It smoothly idles around 950 rpm and the engine temperature is always between below C to 1/4 above C in the temp gage range (temperature is colder the faster I drive).

As long as I maintain the oil level between min and max, will it be okay to continue driving in this manner? Should I seek further investigation by a good mechanic? Will I need to brace for shock and expect a complete rebuild is in order?