I have a 2000 Ford F150 4x2 XL,4.6 V8,auto transmission. It currently has 93k miles on it. I bought this truck used,and I now want to service the transmission. I was told that I should not use synthetic transmission fluid in my truck if the previous owner always used regular transmission fluid ,and not synthetic. I was told by using synthetic it would not be good for my trans. Is this true?