Use the cheapest copper plugs you can find and clean or change them often, its the best you're going to do. I had a Corolla of this era and no snake oil, high mileage oil, additive, or piston soak is going to cure the problem. As others have said, its a coking issue due to too few and too small oil return holes in the pistons. If you've ever torn one down, you'd see the size of the mountain you're trying to climb by trying to cure this problem with different oils or whatever. The carbon encasing the rings will be rock hard, to the point you'll need a hammer and chisel just to dislodge the rings, if you even can. Then many hours soaking the pistons in solvent to loosen the carbon enough to clean things up and drill out the holes a size or two larger. Easier just to use new pistons.
They will run forever if you just keep adding oil, I had one burning close to a quart every 400-500 miles starting just under 100,000 miles, very little smoke at all, and it went 400,000 miles before I sold it to a junkyard due to severe rust. It had an aftermarket cat on it from about 200,000 and still, somehow.. I have no idea how... passed emissions right up to the end. The engine ran fine and had decent power and great gas mileage, it just went through oil like there was no tomorrow.
Cheap copper plugs, don't waste money on anything else.