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I remember this topic came up about two years ago, and one of the contributors did some serious research on the subject (including mailing off oil samples to be chemically evaluated at various mileages). If I remember correctly, he said the Synthetic Mobil 1 oil was still somewhat reasonablely funcitonal at the 10,000 mile mark (in his 700 series Turbo), but the soot factor and parts per million of "insoluables" was getting beyond acceptable values. I think he went back to a 5,000 or 7,500 change interval after that experiment.
If you take care of your Turbo (proper idling cool-down after a drive), I don't doubt the ability of the Synthetic oil to last 7,500 miles. However, weighting the cost Turbo wear versus more frequent oil changes, I usually suggest 5,000 miles (for a healthy car) or 3,000 miles (for a car with more wear and tear).
The following conditions would lead me to advise a 3,000 mile Synthetic oil change intervals (among other factors too):
1. Compression values dropping (more piston and cylinder wear)
2. Turbo lifespan aproaching an end. Example: Excessive shaft play, or it already seized up once and was "unseized" with a 10mm socket (I've done this once on a friends car, and it's still going 6 months later).
3. Vehicle is driven hard or lack of cool-down after driving (lead foot driver or not given proper cool down time before shutdown).
God bless and drive safe.
Fitz Fitzgerald.
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'87 Blue 240 Wagon, 255k miles.
'88 Black 780, PRV-6, 147k miles.
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