Volvo XC90 Owner's Manual page 168

Maintenance and service
Checking and topping up the
power steering fluid
Check the level at every service. It is not
necessary to change the fluid. The level
should lie between the ADD and FULL marks.
See capacities and recommended grade on
page 229.
NOTE! If a fault should arise in the power
steering system or if the car is without power
and must be towed, it can still be steered.
However the steering will be much heavier
than normal and it will require more effort to
turn the wheel.
NST