The M1 is regarded as a 'heavy' 30WT, compared to the Castrol which is is a 'true' 40WT. This is why I ran the old M1 once in my Infiniti (spec'd for 5W30) after seeing good used oil analysis in the VQ35 engine. I myself didn't do a used oil analysis, but the engine sounded very smooth with it during the 7500 mile OCI (very scientific, I know).
I'd like to try out the Castrol, but since this car now mainly does short trips of <10 miles, I'm wondering if the heavier Castrol will take longer to warm up versus the M1 in the winter.