I would suggest the reason would be the lack of airflow across the rear cylinder, which causes that cylinder to run hotter, which is not an issue with the M3W. What's important here is getting the oil hot enough to lubricate as intended and to burn off water which is introduced into the oil via blow by. As noted in this thread and others on the forum who have monitor actual oil temp, the X-wedge does not seem to run abnormally high oil temps. Highest (short term) max numbers I've read are 110-120C or 230-250F which is generally considered normal/safe for synthetic oils.
