Okay guys so I ended up emailing a technician at Valvoline and had a fair decent chat with him!
He sent me a PDF file on Maxlife (which my phone doesn't want to open) and explained that Maxlife 20w50 is high in Zinc and Calcium, and as we all know contains 'special additives' to prevent leaks.
He did however talk me out of using it whilst visiting colder snowy areas and recommended Durablend 10w40 because the start up viscosity of the Maxlife is double to what Ford recommends. He also didn't recommend a 20w50 below 5c (40F) which is much warmer than what a lot of people have run it at, but then again when I ran it in winter last year it did turn over very slowly when the temperature was around freezing point compared to a 10W, but he did say I can run it year round in my climate without issues, and 10W40 would be good year round also.
I have a feeling Maxlife oils are always on the heavier side of the stated viscosity too!
I'm going to chuck the 10w40 in before I head off, as well as 1/2 bottle of MoS2, and if the weep on the front doesn't get much worse then I'll stick with 10w40 year round and reap the benefits of a thinner oil!