You can run the synthetic Amsoil DEO 5w40 year round. when choosing oil the number before the w is the cold rating. the second number is the lube rating after warm up so say if they both end in 40 they have the same lube properties when the engine is warmed up
I believe that the engineers at Ford/International had a good reason for not recommending 5-40 for anything other than below freezing temps.
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