Edit: Didn't see morbid's post about the function of the lower bumper panel.
my physics could be off on this, but wont that trap heat in the engine?
Nope, that lower panel performs two functions:
1. It keeps air that enters through the lower bumper opening flowing
through the radiator, instead of wasting through the lower resistance path below the radiator that's created when the panel isn't in place. On the CSVT's, Ford put foam gasketing on this panel where it contacted the lower radiator support.
2. This panel works somewhat like a proper full sized splitter, in that it creates a low pressure zone
behind the radiator, which helps pull air through the radiator. Ford and other OEM's have done this on a lot of cars, even a simple 1 or 2" high vertical strike on the lower radiator support will create this low pressure zone behind the radiator.
On your oil weight question, if anything, higher viscosity oil tends to have higher pumping losses, it has higher shear energy, and it tends to dwell in the bearings longer, all of which will raise the oil temps, so I'm surprised to hear you have lower coolant temps with heavier oil, especially since you're dumping oil heat into the coolant. Perhaps the environmental conditions are different since you changed the oil?
Do you have decent full sweep oil and coolant temp gauges, or is all your information coming from the OEM dash gauge? Reason I ask is that I do have good oil and coolant temp gauges on my (non-boosted) CSVT, and I can tell you I've learned that the dash gauge is about useless as a temperature diagnoses tool. My point being that we need to put some real temperature numbers to this problem, otherwise, it's difficult to diagnose where your problem might lie. I have two well instrumented boosted track cars, and even with all the gauges, I sometimes have to hook up a data-logger to get at the root of a problem.
Others have touched on tune, and this can make a huge difference in engine temps. Lean fuel and/or advanced timing can drive up coolant temps in a heartbeat. What's your WBO2 telling you about your tune?
One other thing to check, your coolant's anti-freeze to water ratio. Among all the basic thermal properties of water and water/glycol mixes, the most important is the thermal convection coefficient (TCC), expressed in Watts per Meter per ºC, this is primarily how heat moves between the coolant and the engine or radiator surfaces, assuming turbulent flow and no localized boiling is going on. Pure water has
twice the thermal convection coefficient of a 50/50 water/glycol mix, and the more glycol, the worse it gets. Depending on Vegas wintertime temps, you can probably get by with a lot lower glycol concentration than somebody in Minnesota.
Water Wetter: WW is good for perhaps a 5% improvement in TCC with a 50/50 water/glycol mix, and with pure water, less than 5%. Compare this to doubling TCC going from a 50/50 mix to pure water. However, WW is a good additive for pure water cooling systems due to its corrosion inhibitors, anti-foaming and surface tension reduction agents, and when you blow a cooling system on the race track, unlike glycol antifreeze, WW won't leave the equivalent of a patch of glaze ice for the bloke coming up behind you.