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My turbo makes very similar power to yours, except mine is strictly a track car. I run the Valvoline VR1 20w50 oil also. I have used synthetic oils, specifically Mobil,but my oil consumption went up significantly, and the oil pressure would drop when it got hot, so I went back to the VR1. I had a similar discussion with one of the Lindsey brothers a few years ago, (before they started carrying the Brad Penn oil), and they found the same thing with the synthetics at the time, and they also went back to the conventional racing oil, for their turbo, at the time. Personally, I would not go to a lighter oil for the summer. I also use VR1, in my 928, with over 240,000 miles, including many track miles, and this motor still doesn't use oil, and still runs strong.
On the other hand, we have a rebuilt SP2 NA motor, which was machined with synthetic oil in mind, and use 15w50 Gulf Competition oil, because it has plenty of zddp. This motor is over a year old, and still doesn't consume oil, and doesn't seem to have lost any power yet, and runs continually from 4000 to 6500 rpm.
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