My reply focused on the original question from afisk. He's not asking about a new rebuild break-in. He’s way past that. There are so many threads here on opinions on oils and additives that it is hard to find the one I got the MB Club of America slide show from, but I am sure they did their due diligence. I did find
http://www.sl113.org/forums/index.php?topic=17699.0 In reference to the additive adequacy contemporary oils.
"After looking at their product guide I would agree that their 15 W 40 is the one to use. At 1300 PPM zinc additive, you should be fine with that."
It’s probably a moot point since afisk’s question is: "For some time the engine has burned oil. I am thinking about using the high mileage oil and also putting in an additive that is supposed to build up around the rings to allow for a better seal. Do any of you have experience with these additives? If so, what are the downsides to using them and is it a good idea to use them in an older car? My other question is whether or not to put in the zinc additive when changing the oil. I want to use a 10 W 40 oil and mix it with the zinc. Does the engine need the zinc additive?"
My point is its too late for zinc or an additive to build up around the rings. My experience is: save the money for a rebuild. There are no
effective additives for a long-term oil burner. The rings are probably so worn they have no pressure left to seal. If they could seal, the gunk deposits have probably turned to carbon behind the rings and filling the grooves to where the rings can’t roll in their grooves to effect a good seal. Some slight sealing improvement can be found using heavy oil 20 W 50, but it does not protect as well (see the grafts in the slides) at normal operating temperatures, so the wear might even increase. 10-40 is probably the best compromise. Change it rather than just topping up and just keep burning clean 10-40 until he can pull the motor and rebuild it.
I've driven a few oil burners when that was all I could afford to drive. I tried 8 or 10 remedy products and none made a worthwhile improvement. In all cases, it was a waste of money and time reading labels with ridicules claims.
He asked about additives building up the rings. Visualize out of round, and non-cylinder surfaces. Visualize the piston hole, the piston and the rings are all worn a few thousandths all the way around: 0.005 on one side, .005 on the other, the hole is too big by the same. Worse, the ring has lost the spring shape and can’t push on the walls any more. And the ring has deformed and worn the groove: the ring is too small and the groove is too big. The rings are harder than the aluminum piston, but still, the rings are cupped top and bottom. When you have surfaces that can’t hold the pressures of combustion, oil molecules are not going to help. Very tiny beads might fill the gaps , but only until they wear the gaps bigger. Additives are not going to fill the gaps. On top of all this the gunk ahas burned around and behind the pistons cementing the rings in and blocking any oil from coming throught the piston to th oil rings.
I've even tried flushes and kerosene. (I got a big chuckle once when a friend of mine had flushed his motor with kerosene. It would not start, so he had me turn the motor over while he held a plug wire near the dipstick to see if he had spark. The explosion blew his hat off along with his eye brows).
The flushes only remove oil from the friction areas and accelerate the wear while you run the motor with them in the oil.
My experience is save the money for a rebuild. There are no
effective additives for a long term oil burner. Some slight improvement can be found in heavy oil 20 W 50 but it doesn't protect at normal operating temperatures so the wear might increase. I've drive oil burrners when that was all I could afford to drive. I tried 8 or 10 products and none made a worthwhile improvement. In all it was a waste of money and time. He asked about the rings. We've seen plenty of oil burning piston & ring sets, they are often so gunked up that aditives can't get in any way. I've even tried flushes and kerosene. (I got a big chuckle once when a friend of mine had flushed his motor with kerosene. It wouldn't start, so he had me turn the motor over while he held a plug wire near the dipstick. The explosion blew his ht off along with his eye brows)