Yeah, there are HDEO 20W50 and they would be extremely shear resistant. Too thick for 95% of applications but very shear resistant. There are also some 15W50 HDEO. I remember once seeing a 25W60 on the market in Australia, I think.
Those heavy oils are only useful in very hot climates or for an engine that runs hot for long periods of time, i.e.long haul truckers, marine, generators, construction, etc. Very bad for cold weather, short hops or stop-n-go where the oil never has a chance to warm up and thin out. Thick oil doesn't flow and tends to put the oil filter into bypass more often, or all the time in some cases. Running an oil too thick for the ambient temp and operating environment results in more unfiltered oil being run thru the engine. I know exactly when the bypass opens on my gas truck, by oil temp and pressure differential (I have two pressure gauges, one on each side of the oil filter to measure pressure differential) and know the exact bypass setting of the filters I use. Unfortunately, I have never found the bypass spec for the IDI (the bypass is built into the filter head rather than in the filter like most engines) but that's not for lack of looking. I do know, generally speaking, that the chances where you can bypass the oil filter come at oil temps below 150F and at higher rpms. In other words, when the engine isn't fully warmed up, don't work it hard or run higher rpms if you can help it and you minimize filter bypass. Because of our oil coolers, which warms the oil as well as cool it, when you get coolant temp up, oil temp follows fairly quickly. In my experience, the coolant temp is at normal (for the ambient temp) within 5 miles of steady running on a day that is above about 50F. Within another five miles, the oil temp is usually there too, or close, so within 10 miles of steady running, the oil temp is at around 180F. You are safe from bypass there. One reason emergency vehicles have shorter engine lives than some is that they go from cold start to WOT without a warmup, so the filter is bypass all the time. That's why some emergency vehicle operators use block and oil pan heaters year around.
All this is one reason an oil temp gauge can be very useful in choosing an oil viscosity. You monitor operating oil temps and choose an oil that matches the oil temp. Remember, oil doesn't reach it's rated warm viscosity until it's at 212F (100C). A 40 grade is a range of actual viscosities measured in Centistokes (cST), which is from 12.5 cST to 16.29 cST at 212F for 40 grade. An oil can be call a "40 grade" when the warm viscosity grade falls within that range and some oil's "blueprint specs" are at the thin end and others at the thick, not accounting for variations in batches. That same oil at a cooler temperature is much thicker, so a 40 grade at 180F could be as thick as a 50-60 grade oil at 212F and not flow well at all (and bypass more often).
In my own case, the way I now use the truck, the oil temp on my old '86 seldom (basically never) gets to, or past, 190F. The water-to-oil oil cooler warms it up in 10-15 miles of driving ( the range to account for ambient temp) to approximately the same temp as the coolant but if I won't work the truck hard or run for long distances hours at a time, it never gets any higher and is usually lower in cold weather. At the 180-190 range, a standard 15W40 is running at the middle to upper range of a 50 grade, not to mention cooler weather. A synthetic oil with a high Viscosity Index (5W40, for example, a higher viscosity index is an indicator of an oil where the viscosity is less effected by temps) will do better in this regard and will flow better, but I don't use them due to the extra cost. My cost effective answer is 10W30 (Rotella T5, a semi-synthetic) which operates in the low to middle 40 grade area at my normal temps but flows well cold or cool and is very shear resistant.
You can do a quick-n-dirty check with the oil pressure gauge (must be a "real" one that is undampened). You make a cold start, the pressure is much, sometimes VERY MUCH higher. Pressure is resistance to flow. As the oil warms up, it thins out and flows better and oil pressure goes down. If you have a real gauge with numbers on it, you can get a rough idea on whether you are using the right oil grade if you start keeping records. It will take some time and a season to do, but here goes. In the summer, run the snot out of the truck for a decently long period, at least 50 miles to be sure, or a long grade in hot temps, long enough to get the oil temp very hot. Under those circumstances, that is likely to be in the 212F range (might be higher so if you live in AZ, use common sense). Record idle oil pressure and at 2200 rpm (the stock oil pressure rpm spec). Then start recording oil pressure at idle and 2200 in a variety of conditions, cold start, short hops and most importantly, at the times when you are driving your truck as you normally do, the way your truck operates MOST OF THE TIME. If you see much higher NORMAL pressure readings than your HOT readings, you may be running oil that's too thick for the temp and operating conditions or with too low a Viscosity Index. You can do a better job if you can also measure oil temperature.
Shearing isn't really much of a problem for most of us in the IDI crowd (based on my own UOA and others I have seen from IDI). As I said earlier, the HPOP in the PSDs tends to chew on the oil a bit but after looking at more than a hundred oil analysis on them, the interesting thing is that they shear down to a 30 grade and that's it. If you install a 30 grade to start, it doesn't shear down any more.
