The s stands for sport, but what it means in the context of the 3.0 is that the E70 has a higher output 3.0 engine that the last E53 3.0, as it is a newer generation Valvetronic engine. There isn't an E70 3.0i, which means that the s is somewhat redundant.

The same s designation is used on the X3 3.0i and X3 3.0si (although the si isn't available in the US), to designate a higher output version of the same displacement engine.

With the E53, the 4.8 was an s, whereas with the E70 the 4.8 is not an s.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue