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Is it time? Is the oil due for a change chemically?
You can send a sample of the oil off to a lab for about $15 and have it analyzed to see how dirty it is and if it's due for a change. It might be worth doing, given that you're putting synthetic in and changing it as often as most people change standard oil.
I ran part synthetic on my 740T, and felt like it needed to be changed every 3-5K. Full synthetic may last longer, but on a turbo, the main concern for my changing my oil is particulate contamination in the oil - soot.
Either way, my parents also had a V6 ford pickup. It didn't leak oil, and the quick oil change place that last changed the oil had air-hammered the drain bolt in. 2 years on the same oil, and my mechanic had to remove the bolt by welding on a new one and using a 6-foot breaker bar. Six foot!
The car was never obviously the worse for wear from it, and we had it for another 5 or so years. Point being that oil change intervals are often chosen by what seems right to people, and not scientifically. Every 3,000 miles, or two years (there was a study about taxi-cabs and their oil changes vs engine wear) - it's up to you.
Cheers
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