I don't know...Our 1992 Range Rover for a while was using almost a quart every 500 miles, with absoutely zero smoke from the tailpipe. No check Engine light came on (although it did work) and the thing ran like a dream on the road. Always started, no fouled plugs, etc. The thing did stink when it ran like some old carburetted car, though. We used Lucas additive for "worn, high mileage, oil-consuming engines." That alone actually reduced consumption to a quart every 1200-1500 miles.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro