[quote author=bjowett link=board=2;threadid=6644;start=msg54387#msg54387 date=1066935963]
Use a good synthetic, change it when ever., 15k is a nice number.. just change the filter and add a quart every 5 - 7k. I've had several high mileage engines open that used synthetic - spotless in there.
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You should not be pushing any oil to 15k without performing Used Oil Analysis. Generally, even Mobil 1, Redline and Amsoil (the three best) crap out between 8,000-12,000 miles. By crap out, I mean the additive package is depleted (TBN), often the oil is oxidized, it starts to thicken out of grade, and you see a spike in wear numbers such as lead, iron and aluminium.
How long an oil will last is a function of several item: Outside tempuratures, length of trips, condition of the engine, how stressed the engine is, quality of oil, oil capacity of the engine (more capacity, more additives to deplete), how hard you drive, and others. I do not recommend exceeding 7500 miles per change on Synthetic. If you live in a warm climate and drive only freeway miles, 10,000 would not be out of the question. If you live in Alaska, only drive two miles each way per day, tow Junk's trailer, and it is winter, 5,000 is pushing it.
Cary