I have owned 4 Durangos, one with 360/5.9 and 3 with Hemi (5.7). I have towed with all. The Hemi is a huge improvement over the 5.9 in tow power and fuel mileage. I have towed up to ~8,000 lb (carefully) with the Hemi and ~6,000 lb (fast). In a 3/4 ton PU, 10k# should be fine. A NA engine will need to spin a little to pull the hills but within reason does not hurt anything. Hemi's like to spin! When towing, I like to limit to about 4,000 RPM on a full effort pull to preserve the engine... though higher RPM would top the hill faster. Also, with a significant hill ahead, I gear down on the flat to save the heavy shift on the hill. With the 5.9 was more like 3,500 RPM max practical.

My current gen III Durango has over 170k and runs perfectly. Oil consumption as new. 5w20 Mobile 1 since the first oil change. Older hemi's I ran with 5 or 10w-30 but not with MDS.