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.