Been running 10W30 Rotella or DELO in mine for decades. The only times it hasn't had 10W30 are in the days when the 10W30 diesel rated oil was unavailable in my area at various times. Straight 30 grade or 10W30 was the earliest recommendation before 15W40 took it's place in about 1986. I would second the caution that you might want to run a higher grade oil in particularly hot weather or rough towing duty but I can say that 10W30 didn't kill my engine towing at 18K GCWR across the country at various times in the '90s. And it's had a Banks turbo on it since almost day one. A few years back, I had 15W40 in it and hooked up my test oil pressure gauge and measured oil pressure at several rpms with the oil at about 170F. SHortly after, I changed the oil to 10W30 and checked again, making sure to measure at the same oil temps. I saw only about a 4-5 psi difference in oil pressure at 2000 rpm so I quit worrying.
On top of that, I found my engine runs a pretty low oil temp and think most others do too. I hooked up a temporary oil temp gauge (it replaced the dipstick) and found the oil only running 170-180F 95 percent of the time and never breaking 200F any time I had the gauge hooked up no matter how I worked it (to be fair, I'll disclose that I never did a 100 percent workout with the gauge hooked up either... maybe 75 percent at best). Since oil viscosity is a product of temperature, and since oil viscosity is rate at 212F, at any time you are running the oil below that rating temp, you are running thicker oil than you need. At 180, a 40 grade could be running in 60 grade territory and my 30 grade is well into 40 grade territory. If your oil temp never goes past 212F, there are only a few circumstances where you would need a 40 grade. A tired , well-worn engine might be one of them. I my case and in my operational scenario, I verified the 10W30 works fine.
The benefits? Much better cold starts and probably a slight bump in fuel economy. Maybe a little more power. The downsides only occur if you work the engine hard enough to get the oil really hot and keep it there long term. The diesel rated 10W30s are almost 40 grades anyway (very high viscosity in grade) so you get a lot of protection. The formulations (additive packs) are about the same in a family (e.g. Rotella or DELO), but the base oils are thicker in the heavier grades. The other downside to thinner oil is protection from fuel dilution. If you have a dribbler, slobberer, a short hopper or operate a lot in cold weather without taking some precautions, you could end up with a lot of fuel in the oil that could reduce the viscosity by dilution. That's not really an oil issue but one of care and maintenance but it could be a problem for some folks (a remote possibility since most diesel rated oils are deigned to deal with fuel dilution).