My '07 F150 SC has 31K+ miles on her. All highway. I pull a 6K lb. camper. I'm going to have the transmission flushed. I know the MerconV is a blend. Should I consider a full synthetic? And if I go full synthetic does that void the warranty? Should I also have both differentials and transfer fluids replaced? And there again consider full synthetic? I thought the diff fluid, 70W140 was a full synthetic. Or should I consider 75W140 full synthetic?
I love my truck and want to 'do her right'.

 

I would always recommend full synthetic for everything and I think most everyone here will agree, it does not void any warranty.
According to the Haynes manual, you should only have to change the differential and transfer case lubricant every 60,000 miles or about 48 months. Of course, if you are going to have the truck for the long run then changing it over to synthetic might reduce wear and tear and possibly prevent some problems you may have with them later down the road. However, I still think I would at least wait until 60,000 miles.

 

MERCON V is a blend only if you buy a blend. If you buy a full synthetic MERCON V then it isn't a blend. For example, Mobil 1 synthetic ATF is MERCON V.