I believe that since the oil is thinner the engine gets up to operating temp. more quickly and is able to burn off more fuel in a short distance. German Castrol is thicker than most 30 weights in freezing temps even though it is a 0W30, which might be the reason for the high fuel dilution in the past.
Of course there are varying views on this and some people told me that a thinner oil would not help. We didn't get really cold in NJ until late January, so that could have been a factor.
The runs that showed fuel dilution higher than .5% were all during winter on GC.