The external filter does provide more protection than the standard internal filter, and is so much easier to change! While the filter is off, put 5 gallon bucket under the filter (helps if you did what I did and mount it behind the bumper and with the lines pointed down while the filter is off). Start the engine with the filter off, and it will drain the transmission in about 3 minutes at idle. then taking off the pan becomes very simple, no dripping fluid on you! You can then change the in pan filter, and more important, clean the in pan magnet too! It will pick up some metal debris while the engine is running, and it sticks to the magnet for years, but can not be cleaned out without removing the pan.
I also have a engine coolant water filter too. Right next to the trans filter. Coolant filter is Napa #FIL4070 with a kit to install it Napa FIL4019. It is plumbed into the heater hose line, and new filter installed every September, so that I have good water flow all winter, it will slightly plug in the summer, and reduce heat in the dash A/C system!
I can understand the parts guy saying that a external filter 'can' reduce transmission oil flow volume through the transmission coolers. And it will reduce the flow - say from 10 gallons per minute to about 9.5 gallons per minute. so just a small amount. But well worth it to keep the fluid very Clean! Just change the fluid filters every 10,000 miles (even if you are not changing the fluid that frequently) and keep an eye on the transmission temps. If the temps start to climb, then it is time to change the filter again!
Happy camping!
Fred.
