It has zero effect on fuel consumption, go to a thinner oil if that is the objective.

It also doesn't matter how hard it is driven. The problem is additive depletion and subsequent oil breakdown at much less than the nominal 15,000 mile interval. As long as you change it much more frequently the engine should be fine but I fail to see any upside to the experiment.

May want to try imported air in the tires (reminds me of a Blondie song) to see if it has a measurable impact on fuel consumption.

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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
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2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue