Honestly, for me, I think its more a fear of the unknown running 10W40 in my Zetec vs what Ford says I should run. I've been running 5W30 in my Zetec for 5 years - car has 81,000 on it and will be ten years old in April with the original oil and water pump. I would not run 40Wt in my motor because I don't see a need for it when Ford says 20Wt is best. If I was racing the car I would use a higher WT oil, probably 40Wt, to keep the oil from being too thin at constant high temps.
PS, Tom your base tune really woke up my car, thanks again.
My ride is 10 years old this year, I might bake the little hatch a cake!