As someone running two Ford Modular V8s in extreme heat conditions (125+ ºF during daytime and 105 - 115 ºF at nighttime), here's my two cents.
Ford recommended 20W-40 in Middle East export engines up until 1995, when it discontinued Motorcraft 20W-40 and the recommendation was then changed to 10W-30 - the thickest grade of gasoline engine oil available from the Motorcraft brand up until 5W-50 full synthetic hit the shelves not that long ago. People have run various oil grades in Modular engines, including 15W-40 HDEO and 20W-50 of different varieties. For some people, 10W-60 and 20W-60 was the oil of choice. At the local dealership, 10W-30 is still the default standard for Modular V8s. I have even seen some folks requesting 5W-50 from their service advisers.
The 4.6L in my Expedition is bone stock, while the 4.6L in my Grand Marquis has a number of different modifications done to it and does, in comparison, run quite a bit hotter than the Expedition. The Grand Marquis is equipped with a 180 ºF thermostat, and the electric cooling fan was programmed to come on full blast when coolant temperature hits 206 ºF (stock form was 212 ºF). The Expedition has a clutched fan. Both run on Motul 8100 X-Cess 5W-40 with a Fram Ultra XG2. Coolant wise, they're running 30% G13 to 70% distilled water. A 50/50 ratio may provide a higher boiling point, but it also causes the coolant to run between 15 – 18 ºF hotter. While I cannot get engine oil temperature readings on my ScanGauge-II's that both vehicles are equipped with, cylinder head temperature and coolant temperature is something I continuously monitor, and base my driving style on.
As the Grand Marquis is modified, I will limit my observations to the bone stock Expedition.
In general, coolant temperature readings are 2 ºF cooler than cylinder head temperatures. On any given summer day, coolant temperature at startup, after the engine has been sitting overnight, will generally be in the 105 - 120 ºF range depending on time of day. Operating temperature is often reached within 5-8 minutes, if not sooner (if you're firing up the engine past noon). If you have driven the vehicle and parked it for a couple of hours, 8 hours in my case when I'm at work, coolant temperature at startup will be in the 115 - 130 ºF range at ~17:00 hrs.
Speed is a major factor when it comes to coolant temperature – go too fast, and higher engine RPMs will generate more heat. Go too slow, and decreased air flow through the radiator will generate more heat.
During the wee hours of the morning prior to 10:00 hrs, and after 17:00 hrs in the evening, ambient temperatures are between 113 – 120 ºF. If you’re on the road at these times, with the AC running and cruise control set between 50 - 60 MPH, coolant temperatures will generally hover between 206 – 211 ºF. Between 60 - 75 MPH on cruise, you're looking at 211 – 218 ºF. Once you hit 80+ MPH, you’re up to at least 222 – 226 ºF. If you’re stuck in traffic, the lowest coolant temperature tends to be 213 ºF and can go right up to 230 ºF – sometimes higher. I generally find myself turning off the air conditioning for a few couple of minutes at a time, until coolant temperatures drop, before turning it back on again – Hüper Optic’s X3 tint is a lifesaver at this point.
If you happen to be on the road between 10:00 – ~16:00 hrs, ambient temperatures will often exceed 125 ºF. I prefer not to experiment with coolant temperature readings at different engine loads/speeds at these times, and rather maintain a speed of 55 – 60 MPH on cruise. Coolant temperatures will hover between the 215 – 220 ºF mark at these speeds, and depending on traffic, may raise to between 224 – 232 ºF for several minutes at a time. I prefer not to turn off the air conditioning during these hours, but with the AC clutch disengaged, coolant temperature drop much quicker to an acceptable (212 – 215 ºF) range.
Motul 8100 X-Cess 5W-40 if a full SAPS oil meeting A3/B4, LongLife-01, MB Sheet 229.5 and VW 502.00/505.00 in addition to Porsche A40. It has a HTHS of 3.7 cP and a flash point of 446 ºF with a NOACK is 9.9%.Viscosity at 100 ºC (212 ºF) is 14.2 cSt.
In comparison, according to
Motorcraft Chemicals and Lubricants U.S. Catalog Motorcraft Synthetic Blend 5W-20 has a viscosity of 8.7 cSt at 100 ºC (212 ºF) and according to a
PQIA test done on 03-06-2017, a NOACK of 14.6%. HTHS will be < 3.5 cP, which I recall to be around 2.6 cP.
Given the temperatures my engines see, I would not want 5W-20 (even as a full synthetic) in my sump. YMMV.
