No way to quantify the stress.
Gas mileage will have more to do with driving style than anything else.
If your mech thinks it is the OSV then have him fix it. That is cheaper than if it is valve seals. Worth doing it first.
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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
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