The modular engines have a large number of extremely small oil passages in the heads. This is a byproduct of all the individual valves and valvetrain that has to be lubricated....plus the cam journals. Because of the large number of passages...If these passages were made larger in diameter to accommodate a higher viscosity oil, oil pressure could not be maintained at an acceptable level when the engine was hot.
When the modulars first came out in the early 90's Ford actually recommended a higher viscosity oil. However, customer's experienced a large number of cam journal failures that was determined to be the result of oil starvation. This damage required replacement of the heads since the modulars don't use replaceable cam bearings. In the mid to late 90's Ford sent out a revision mandating lower viscosity 5W20 oil in all modular engines. Ford later changed this recommendation to all of their engines but I suspect that decision was based also on achieving better corporate fuel efficiency to meet federal guidelines.
The quality of oils today is such that lower viscosity oils protect you engine just fine. They are much better than in years past.
You should not change oil viscosity on this engine. If the engine is fully broken in...I would, however recommend running a quality synthetic oil such as Mobil 1, Belray, Royal Purple, Kendall,or AMsoil.
HTH
Kerry
