...no issues with mine, sounds like the salesperson who delivered the message from the mechanic either was misinformed, and if not...then the "mechanic" needs to read up on his profession... ...I haven't towed yet, but will next week and am hoping to get an idea with a 255 mile tow through the mountains of WV, TN, VA and NC...
If you read my post a couple up, you'll get the info on messages one will get..I'd say if you let the SAME DEF sit in your tank for, oh, over a year, you "might" have an issue, but I even doubt that with a shelf life of a year on the container purchases.