Hi.
It really doesn't matter. It's out of warranty, and both will provide protection for your engine.
Fuel economy is a toss-up. In real world conditions, you won't notice a difference.
Performance. Again, it's minute because your engine won't differentiate between 20 and 30.
Protection. Well, if Ford recommends an XW-20 in the manual sure it's ok. The engineers have done their homework to make sure running a lower viscosity oil will not "hurt" the engine. But remember, the beef of this decision lies within the fact that better fuel economy and efficiency numbers can be posted in a lab, submitted to the government, etc etc. I mean Honda does it too so why not right?
Now. A thinner viscosity oil, due to simple physics will not provide the same high temperature protection compared to a higher viscosity oils. It's really simple. Higher viscosity = better protection at higher temperatures. However, it could be providing protection OUTSIDE of most consumer's driving needs or most engine operating conditions.
Will it hurt to use a 30? No. Will it hurt to use a 20? No. Should you run a 30? Sure. Should you run a 20? At least always.
Engine tolerances and all that mish mash is irrelevant. The movement to a lower viscosity oil is to be more marketable both to the consumer and to the government. By touting extra fuel efficiency numbers that were obtained in a lab, manufacturers have more marketability in their products, because 1.) consumers want fuel efficient vehicles and 2.) the government wants you to build fuel efficient vehicles.
Remember, it's easier to change a viscosity than to change even the smallest thing in engine design...including microfinishing, which on a scale of the big automakers, will take a large chunk out of profits. Although we do it to our race engines like they drink gas, on a large scale, it's not as profitable. So, the manufacturer just has to test the engine within specified operating conditions, and as long as it meets protection that again are specified as standards (or acceptable), then it would be ok to recommend to consumers.
Plus, the market statistics show that people now only keep their cars for an average of 5 years. It's not like before. So, it may be far-fetched to say this but in my opinion, shorter potential consumer ownership = more lax engine protection requirements.
Anyways, that's me schpiel haha. Probably more than what you wanted but sometimes I get carried away...hahaha
So yep. Either oil is fine really.
