2011 SR5 towing - 5,150 lbs
I pulled a 2,100lb car carrier with a 3,050 lb car with a 2011 4runner.
I started at 1,200 ft above sea level (MSL) and climbed up a 7% grade to 6,000ft MSL
I had 300lbs of tongue weight, so the truck didn't sag very much. On flat road, it would keep bouncing from 4th to 5th and back, so I left it in 4th.
Climbing the grade, I manually put it in 3rd, and it did okay up to about 3000msl (maintained 50mph), and above 3000msl it struggled to maintain 45mph in 3rd. It would drop into second below 45mph and would climb up to 60, then go into third, slow down to 45 and repeat the cycle all over again.
I was getting 12mpg going uphill. It stopped just fine. The trailer had surge brakes and I didn't feel I had a stopping problem, but I drove very conservative and gave myself a lot of room.
Overall, I thought the vehicle towed okay. I haven't analyzed the oil, but I would agree with tlflow that consistently towing this weight would be hard on the engine. The length was 22 feet and I had no "sway" issues. I can't understand why the Tacoma with the same engine can tow 1500lbs more, as I was completely empty and the truck struggled uphill. I am definitely going to install an aftermarket tranny cooler.
For a V6, it did alright. I would feel comfortable towing 4500lbs and having my wife and kids in the car, and camping gear, etc. You will get bad mpg and struggle uphills, but if you want a vehicle to tow with, buy a Chevy 2500HD or a Suburban.
I am looking to get a 19-21ft boat and I wanted an SUV w/ good MPG, could go hunting, fishing, camping, etc and my wife could still drive around. We looked at a Tahoe (ok MPG on hwy, excellent towing, but VERY expensive), a highlander (I wouldn't be caught dead hunting in that thing), and the SR5 (we have 4 kids and needed 7 seats). I really wanted a Trail, but the geniuses at Toyota didn't think anyone with over 3 kids would want to go offroad.
A 21ft boat is about 3000lbs and the trailer 1,200. That is the max I would feel comfortable towing with the SR5 and I still am going to need a tranny cooler and a tranny temp gauge.
BTW - does anyone know of a tranny cooler I could install?