I have to agree with those that suggest that this 18 year old engine is just plain worn out. While oil rings and compression rings are different parts, I suspect that a compression test would confirm that it’s just plain old tired.
You could try some oil system flushing additive. Follow the instructions on the bottle to the letter. In addition to wearing, rings also lose their spring tension over time. If they’re also gummed up then flushing them out might help. It’s a longshot, but a cheap try worth trying.
While heavier oil is worth a shot for lost oil pressure at idle, I’m not sure it’d help something burning that much oil. But, with respect to the engine’s top end needs, I’d be inclined to try it. You may have so much wear now that it can get into those tight spaces and lube the top end sufficiently.
Additives in general are a bad idea for engines that are running well, but for an old engine that’s worn oout anyway they’re worth trying.
In summary, flush it out and try a heavier base weight oil. Post back to let us know how you make out.