I have a 140,000 miles on my 99 XJ8 and it never smokes but I am going through a quart of oil every 300-400 miles. No oil on the ground where the car is parked either have not gone to the service center yet just figure it would be valves and or rings and expensive.

Any help is appreciated?

You might try one of the engine 'restore' products but sounds like rings/valve seals. I see compression/leak down tests in your future.

Unfortunately, a rebuild of your engine is not generally available. Re-manufactured engines are available but pricey.

Just a possibility - a weep from rear cam cover gasket dripping onto exhaust manifold at rear and burning off?

Jim