New rules defining the cold-flow requirements of SAE viscosity grades (SAE J300) became effective in June 2001. The auto manufacturers were afraid that modern injection systems might allow the engine to start at temperatures lower than the oil could flow into the oil pump. [Which would increase cold start wear.] Consequently, the new rules had a thinning effect on oil.

Thinner oils have less drag, and therefore less friction and wear. Right? Perhaps in the test engine or engines that experience normal operation. But somewhat thicker oils may offer more protection for more severe operations such as driving through mountains, pulling a boat, dusty conditions, short trips, high rpm, overloading, overheating and overcooling. ["May" = guess. If you are driving in constant severe conditions, take appropriate measures. MOST Mustang owners are not in constant severe conditions.]

Temperature has a big effect on viscosity and film thickness. [Same is true at cold start ups.] As a point of reference, one SAE grade increase in viscosity is necessary to overcome the influence of a 20°F increase in engine temperature. At a given reference point, there is approximately a 20°F. difference between viscosity grades SAE 30, 40 and 50. SAE 20 is somewhat closer to 30 than the other jumps, because SAE 30 must be 30°F higher than SAE 20 to be roughly the equivalent viscosity.
In other words, an SAE 20 at 190°F is about the same kinematic viscosity as an SAE 30 at 220°F, which is about the same viscosity as an SAE 40 at 240°F. .....
If an SAE 50 oil at 260°F is as thin as an SAE 20 oil at 190°F, imagine how thin the oil film becomes when you are using an SAE 5W-20 and your engine overheats. When an engine overheats, the oil film becomes dangerously thin and can rupture. [Your engine is overheating why? With the properly maintained cooling system and the quicker heat dispensing ability of 5w-20 vs 5w-30, there should be no reason your Mustang is overheating. And higher viscosity oil retains heat longer - so that counteracts the cooling system.]

To determine if SAE 5W-20 oils provide the same level of protection as SAE 5W-30 oils, Dagenham Motors in England, one of the largest Ford dealers in Europe, was consulted. [That comment substantiates absolutely nothing. Speculation.]

As wear increases, the efficiency of an engine declines. Valve train wear slightly changes valve timing and movement. Ring and liner wear affect compression. .... Efficiency continues to decline as wear progresses.
Certainly engines that have experienced significant ring and liner wear benefit from thicker oils. Thicker oil use results in compression increases, performance improvements and reduced oil consumption. [True - but its referring to high mileage engines.]

Every fluid is a compromise. Oils recommended by the auto manufacturers seem to compromise protection from wear under severe conditions to gain fuel economy and catalyst durability.

** Thicker oils also compromise cold temperature flow, which may be of concern depending upon climate and season. [Don't ya'll get winter up there in Canada? We sure do here in Oregon and most of the US. Here in S. OR Cascades, its mid 40's(F) on most all summer mornings. Again, cold starts are where most wear takes place. Plus higher viscosity oil takes longer to get the engine up to optimum operating temperature - so the longer it takes to get the oil flowing better to the OHC valve train.]

The best protection against wear is probably a product that is a little thicker .... [Complete speculation - no facts provided.]

The best oil for your vehicle depends on your driving habits, the age of your engine and the climate you drive in, ... [EXACTLY!]