I've been running 10W30 HDEO for many years... year around. I'm talking on and off since 1987 (there were times in the early '90s when Rotella 10W30 was hard to find). With a truck that's been turbocharged since 1987. I've had great oil analysis with it and when I had the engine apart with the blown head gasket debacle, (truck has 140K miles) the cylinders had 0.001" taper or less. The bearings were reusable (I didn't though). Nothing else hard in the lower end needed replacement based on careful measurement. Did replace the rockers with the 7.3L style.
SAE 30 or 10W30 was one of the original recommendations for the 6.9L (all my original '86 owner literature and manuals show it) but was supersceded by 15W40 starting in '87 (which I found starting in the '87 books). I disagree that 10W30 doesn't provide "adequate lubrication." Like any oil, it depends on the circumstances. These are the climate, operational situation (both of which equate to operating oil temps) and the level of wear and tear on the engine.
I ran an engine oil temp gauge on mine for a time till the sender cacked (it was an oddball that I couldn't replace) and the sump temp on my truck seldom approached 200F. As most of you know, oil temp dictates operating viscosity and the actual viscosity grade of the oil is measured at 212F. So if the oil is at 12.1 cSt @ 212F (right at the top of the 30 grade area.. almost 40) then at the 190F the oil temp normally runs, the oil is running at about 14cSt which is smack dab in the center of the 40 grade area ( I determined this by using a temp/viscosity chart). At that temp, a 40 grade is running in low 50 grade territory. When the oil temp goes higher than 212F, then you start going the other direction. The 12.1 cSt 30 grade oil must go to 230F before it drops out of the 30 grade. A typical 40 grade (I'm working off Rotella info on both oils here, other oils are slightly different) at 230 has dropped to about 12cSt, (upper 30 grade) at that temp. A 40 grade must go to 250F to drop out of 30 grade territory.
Another good reason why 40 grade is recommended is fuel dilution. Fuel in the oil reduces the viscosity and it doesn't take all that much to drop a 40 grade down to a 30. Whether this is a problem depends on how you run your truck and the condition of the injection system. No matter what, a short hopper would have more trouble with fuel dilution because colds starts is when most of that happens and it takes heat and running to evaporate it out. When you do freeway type miles, fuel dilution generally isn't an issue unless you have a fuel system problem.
Soot thickening is an issue in diesels no matter what oil you use. As soot builds up in oil, it begins to thicken. It's interesting to see. At some point in the oil change interval, the oil will shear a little... lose some viscosity as the Viscosity Index Improvers (VII) wear out. Shearing isn't much of an issue with a good oil, but there is always a slight drop. Then, as soot builds up, the oil starts to get thicker again. I've seen the 40 grade viscosity in my IH tractor come out thicker than it went in due to soot... and only about 2% to boot!
Permanent shearing is also worth discussing... if only because people bring it up in oil discussions so much. Yes, a 40 grade can shear into a 30 and a 30 into a 20 but does it happen often with a good diesel rated oil in todays world? No. The Viscosity Index (VI), e.g. the span between the "W" winter grade and the summer, is pretty small on most diesel oils. The wider the span, i.e. 5W40, the more VII are in the oil and the greater chance of shearing. Rotella 15W40 has a pretty narrow span and a 135 VI. The 10W30 is close too with a VI of 142. In the old days the VIIs were pretty fragile and that was one reason why straight grades were often preferred for HD engines because in hard use, the VIIs were used up pretty fast and the oil sheared. Those days are gone. VIIs are much better than they were and on top of that, we have synthetic base oils which have much better viscometric performance and don't need as many VIIs. So you 5W40 or 5W30 syn guys don't need to start yelling. Those oils generally don't shear either.
One downside to running a heavy oil at too low an oil temp is more potential for oil filter bypass. Your filter base has a bypass valve. It's designed to bypass oil if or when the oil filter becomes plugged... but cold or cool thick oil doesn't want to flow thru a filter either. Oil too thick is just like having a plugged filter. Cold starts and those first few miles of operation are when bypass is most likely to occur. When the oil filter bypasses, unfiltered oil is pushed thru the engine. A certain amount of bypassing is inevitable no matter what, but minimizing that with the RIGHT oil viscosity for the temperature is helpful to longevity. The one saving grace of our engines in this regard is the oil cooler. Or oil warmer in winter. It will warm the oil as well as cool it, and when I had my oil temp gauge, I noticed the oil warmed up pretty fast. I have oil temp gauges on some of my other stuff, most notably my '05 F150, and the 6.9L oil warms up faster than the 5.4 even though it holds more oil
I can tell you this, 10W30 has worked great in my truck for many years. Towing, hauling and more dyno time than some of you guys put together. I started there because that's what "the book" said. Later, I validated it by determining my operating oil temps. A good way to think about this is, "As thin as possible, as thick as necessary." Too thin and increased wear or engine damage may result. To thick and you waste energy and power. I can't say 10W30 will work for all of you (I suspect it would work great for most but...) but I KNOW it's worked great for me in the places I've lived. If I lived in a hot climate area, especially if I worked the truck hard, I'd likely use a 15W40. I don't see myself as a "thick oil guy" or a "thin oil guy" I just know enough to pick an oil that's right for my truck in it's normal operational cycle and climate. The oil I choose is based on the circumstances. I say this to avoid the hate that often comes with oil discussions. Sometimes I think it would be easier to discuss ****** dimensions than oil. ( : < )