What I have read is the Rover (ex-Buick) V8 was designed from the start to run on very low-grade gas because it would be used throughout the world where "premium" grades of gas were unavailable. Thus the compression of the engine was reduced, and all versions of the engine (3.5, 3.9, 4.2, 4.0, 4.6) were able to run on low grade gas. However, the real reason for the need for premium in these engines is the emissions system, mainly the two catalytic converters. From experience, we have replaced the 2 cats (very$$$) which failed because the previous owner used regular. Regular may also, as mentioned, cause knocking although it would be very light and not detrimental. The O2 sensor (s) might also be affected. Premium is about 20c higher but will cause the engine to run a little cleaner and better, so it is worth it. As for synthetic oil, you might gain 1/2-1 mpg improvement in the engine, but since the engine design is from the 50's there is really no need. The key to longevity is frequent 3,000 mile oil changes or wear on the cam and pistons may result.
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Current:
2005 Land Rover LR3 V8 SE
2000 Land Rover Discovery II SD
1992 Land Rover Range Rover County
Also current:
1995 Mercedes-Benz E320 Wagon
2002 Volkswagen Eurovan GLS
1996 Ford Explorer LTD 2WD
1982 Volvo 240 GL Diesel
Previous Rover:
1995 Range Rover 4.0SE
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