ON my truck I drove it 500 miles before starting to pull our RV.. I also dumped the OEM rear differential oil before I towed and replaced it with Amsoil severe duty synthetic oil.. The OEM stuff smelled like crap and was already discolored..
IN 14 years of towing I have swapped out the rear diff oil one more time and it was actually quite clean and clear and no OEM stink odor... I also run the same gear oil in the front differential..
https://www.amsoil.com/p/severe-gear...4aAn93EALw_wcB
I have run Delo 400 and recently switched over to Costco diesel oil at $26 for 3 gallons... very affordable and made by Warren Distribution... very good oil.. with 149K miles on my 07 Duramax I burn zero oil between oil changes...
I'm not fond of synthetic oil in my engines as I don't drive enough miles a year to warrant the cost and will not leave oil in the crankcase past 4000
miles...