My 95 850 burns about 1 quart (regular) oil per 6-700 miles. Will Mobile one reduce that and be better than regular oil? Also, can the Mobile 1 simply be added, or does the engine need a complete oil change to Mobile 1 ? Thanks.

A quart every 6-700 miles is getting close to being a problem.
You will lose a certain amount of oil from volatility, more-so if you are running hot. In the NOACK Volatility test, AMSOIL 10W-40 loses 4.86%, and others range on up to Valvoline 12.18%. Depending on what you are using now, Mobil 1 could well give you reduced consumption.

Before changing to synthetic, try using a good engine flush. Best quick ones I know of are made by AMSOIL and BG. For a slower flush (couple of thousand miles) look at Auto-Rx. Haven't used it myself, but many good reports.

If you have a mechanical problem, bad seal, bad gasket--not likely that any oil is going to fix the problem.

Mobil 1 is compatible with the non-synthetic. Putting a quart in as makeup will start a cleaning process. However, you'll get better results by making a complete change. If you don't, or won't, flush, watch the oil level. If you see a rather sudden increase in consumption after a couple thousand miles, change the filter and top off.

You'll get mixed opinions on the synthetics in older engines and if the cost outweighs the benefits. Mobil 1 is fully compatable with dino oils and there is little question the synth lubes are superior to regular lubes especially with resistance to extreme heat/cold. There are also a ton of very high mileage engines around that never saw anything but dino oil that was changed on a regular basis and that's the key. I prefer the syn oils and have switched some high mileage engines over without any problem but some guys report seal leaks after a switch. My old 740 was using about a qt dino/1000 miles that actually dropped somewhat on M1. You also get mixed opinions on change intervals with syns. I usually change at 5000 miles but some run it much longer.

bl

My 1996 2.5l 10v with 150000miles has just been changed over to Mobil 1.

My observations:-
There is extra power (or less resistance)....appears to have about 10% extra power based on 40mph in 4th gear...therefore slightly more fuel efficient

The bad news is the cost of Mobil 1 ...lots of people use cheaper fully synthetics and change them more often

Seals can start leaking...some people add a thicker oil to help negate this.
Mine used to burn no semi-syn oil, but now burns a little fully syn, not much but enough to need topping up once a month a little.

It should be fully comaptible, but depends what exact oil you have in there at the moment.