I have both the E53 3.0 and an E70 with the 4.8. Gas milage is way better in the 3.0. (23 hwy vs 18 hwy) and I have had no issue with the oil except when it was new it needed to be topped off once in a while, but not since then. Plenty of power in the 3.0 as well. Unless you're really into performance and want to blast away from the traffic light, I think the 3.0 will be fine. Come to think of it, the 3.0 will blast away from a traffic light nicely too!

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2007 X5 4.8i Space Gray
2001 X5 3.0 Titanium Silver