My issue is the transmission calls for Dexron III in the owners manual and on the dipstick. This transmission was serviced at one point in its life by a Toyota dealership and they put in Toyota T-IV according to a service record.
I need to know if it is safe to do a drain/fill with the proper fluid Valvoline Dex/Merc even though the transmission as the incorrect fluid in it right now.
So is drain/fill fine because obviously during a drain/fill it is not getting out all of the old incorrect fluid that the dealership put in it during a service years ago but it will be diluted with fresh correct fluid.
Basically will Dexron III (Valvoline Dex/Merc) and the incorrect fluid Toyota put in it years ago Type-IV have bad interactions with each other or should I just pull the transmission cooler return line and do a complete flush?.