Just bought 2001 s40 with 88k miles. Transmission is smooth but when I checked the dipstick the fluid color is a kind of dark. I was thinking of changing it. Here are my questions/doubts:

1. The fluid is ATF Dexron III and Mercon oil. Right? What is the best brand to use? Castrol?

2. As I was looking for the fluid at pep boys, i met the service manager i knew. i asked him about the transmission fluid. he mentioned to check the service record. I did not have the service record. he said if the fluid has not been changed ever, it usually got into trouble (?).

but in the haynes manual, it mentioned that:
transmission fluid does not usually need to be changed unless the car is used as taxi ( then it needs to be changed every 40k miles or so). Because of this, i think i can change it anytime. am i right?