Hello, i have a 91 Explorer 4.0 4x4

I know some shops won't change the fluid in a high-mileage trans because they are leery of stirring up all those potential metal shavings in the bottom of the pan or filter, but that's just wrong, do a proper flush and filter change and it'll last longer. Amsoil recommends that on high-mileage trans that have never been serviced that a second filter/fluid change be done a few weeks after the first, this is to catch all the sludge the detergent properties of the new fluid is likely to loosen in the old trans and gunk up the new filter even worse than the old one. Here is their quote: ------------ "Most of the transmission failures after an evacuation (fluid exchange machine) service have occurred primarily on relatively high mileage transmissions that have not been serviced in some time. One reason for this is that the sludge and dirt buildup within the transmission will not completely be removed during the service. When the new fluid (which has detergent properties) is placed in the transmission, over days and weeks, the internal components begin to wash the insides of the transmission. This sludge does finally work loose and settles in the transmission filter, clogging it up even further than it may have been before service. In these extreme cases, where service has not been performed in some time, changing the filter may not completely fix the problem. Some mechanics recommend a second service a few weeks after the first, replacing the filter again, which may be partially clogged due to the cleaning process in the transmission."

Even if you change your transmission filter at regular intervals per guidelines, it's still amazing how much sediment can end up in the valve body. I pulled the valve body on my Ford AODE transmission in part because of an idiot Ford mechanic and improper warranty work years before. Anyway, in every passage of the valve body was a layer of sediment and that included many of the micro holes. Afterwards the shifts were quicker and firmer. Can you imagine the probable sediment in the valve body of a high mileage neglected trans?