I'll throw in my two cents here. I tow a fairly heavy camper, about 9-10k loaded, with my 2007 6.0l. This past fall I decided to try the 10w-30 with Archoil. I towed a couple of times in the fall and seemed to notice a smoother engine and I think it may have helped a stiction issue that was developing in cold weather. Based on what i read here, I decided to go with the same regimen when I changed the oil in the Spring. I towed over a thousand miles in heat approaching 90 degrees with no ill effects other than maybe slightly higher EOT, but not much, if any.
Within the past month, though, I've had a problem. The truck has been hard to start. I fixed a couple of things that were unrelated. One of the batteries was bad, so that was an obvious fix. I still had a hard start problem, so I changed the air filter, which may have helped a little. I think it had some life left in it, but it was on there a while and was kind of dirty so I thought why not.
After talking to a shop and doing some reading, I started to suspect the STC fitting was leaking. When there is a problem with it, hot starts are an issue. This is because the oil is too thin at temperature to pressure up the injectors (mechanics, please let me know if my description is wrong) I decided on Monday to dump the 10w-30 and added 15w-40. It seemed to help immediately. It has been cool for a couple of days, so it really hasn't been tested in hotter temps, but I haven't had a problem starting. It has been idling rough at times, so I still suspect the STC. I'll continue to monitor it for a while.
I think 10w-30 is fine in this engine if the STC is replaced with the screw in fitting, and I don't think it did any engine damage to my truck. It may have even helped some things. I do, however, think it may not work in my truck and I won't put in back in until the STC is replaced.