650 is low idle, that's half of why its 20.
750 would make it closer to 30, change to 30 or 20/50 and it would be mid high 30s.
Reasons for variances.. bearing clearance/applies to failure or or general running clearance, engine oil temp, leakage, OIL WEIGHT.
30 is what I recommend for nice weather.
Extreme ends hot/ cold, 20/50
