I have a 1996 Nissan Sentra with 196K miles. It sat in the driveway for several weeks and when I tried to start it, it didn’t. It sounded OK, but wouldn’t start. I had it towed to the garage and the mechanic said there was a loss of compression. He got it started. The oil had drained from the cylinder walls and the rings wouldn’t work correctly. I can understand this. Old car, lots of miles. Lots of wear.
Can anyone recommend relatively simple things to do to keep the car running? I know I can rebuild the engine, but that isn’t worth it for such an old car. I can junk it, but I like the idea of having an “extra” car. Of course the obvious thing is to not let it sit for weeks at a time. Got that one. 
Is there something useful that can be done just to keep the car running? Oil additives?
Other than an engine overhaul, not much. You can try slightly thicker oil - it used to be common to switch over to 10W40 frm 10W30 - but there’s no guarantee that it will work for you.
Probably the best thing you can do at this point is to just remember to start the car and take it for about a half-hour drive once a week.
some engines if started run just a short time and shut off can experience a loss of compression due to rust building up on the intake valve stems.
Prolonged cranking usually results in this condition going away by itself.
A test called a leak down test will tell where the compression is going, but if it was sticky valves, and the test was not done during the no start condition, the results will be inconclusive.
Change the oil, synthetic oil should help with sticky valves, and drive it a bit more often would be my suggestions.
thanks.
I strongly suspected that those oil additives one sees at the auto store wouldn’t help, but it is good to hear that from experienced people.
If it does sit too long, you can probably get it started by pulling the spark plugs, drying them off if they’re wet from fuel (spray carb cleaner followed by compressed air is ideal, but they would probably air-dry in a few minutes if compressed air isn’t available), and squirting a bit of oil into the plug holes. Crank the engine for a few seconds to distribute the oil, then reinstall the plugs. Chances are it will then start.
At 196K, the funeral is in sight. On the positive side, however, you should be pleased you wrung that many miles out of it. The heavier oil is a logical path, but I’d go a step further and try a “straight” 30W or even 40W. This assumes you live in a warmer climate.
Oil doesn’t normally “drain” from the cylinders to the point that it would cause this, but GaryT’s point about drying the cylinders and plugs is a good one. If the engine floods and sits for a while, the excess gasoline can wash the oil from the cylinder walls and rings. This was more common in the days of carburetors, but it’s possible something’s messed up with the engine management causing it to dump too much fuel into the cylinders.