Don’t underestimate the importance of Volvo’s 2.9 engine cooling system. For Volvo S90 it’s important to have optimal antifreeze inside. Otherwise S90 made in 1997 won’t serve you long enough. In 1998 the company produced vehicles with 2.9 engine having code of B6304G. It’s important to use 10.7 liters of coolant which has Green color.
A lot of B6304G motors are used long in Volvo cars but only with good service. If you choose other than Green coolant or use less than 10.7 liters, you can’t expect normal work. It will be advantageous to follow Volvo’s rules for 1997 S90 service. 2.9 S90 motors of 1997 year will serve more by optimal service.