Either one makes no real difference. They are so close together that I just split the difference and run the 30 weight. 20 weight works good too, I just don't like the volatility, less film strength. I really doubt the difference in wear at startup time is significantly different. If you're really not sure, just run a 50:50 mix, or adjust the mix so your hot cSt of the virgin oil is around 9.8 or 10. 20 weights start out at about 8.6, so the difference between using a 9.8 vs an 8.6 is negligible. Above 9.3 is by definition 30 weight, so we are all around one or the other side of the border. Even I had a 7000 mile run planned this summer, so I'm actually using a 10w30 synthetic.