I had a hard time finding anything called Dot 4+ in auto
stores and just ended up using brake fluid purchased at the
dealer. But, I think the Castrol should be fine.

You might want to check brake pad wear. If the fluid is
low enough to set a light on the dash the pads may be
near minimum size (the fluid level drops a bit as pads
wear).

I would also clean the lid and area around it with a
cloth just to help keep dirt out of the system.

Volvo recommends brake fluid be completly flushed every
2 years or so. If that has not been done it is something
to consider. The reason is the brake fluid absorbs moisture
(you want it to do this because you don't want pockets of
moisture in the line). The fluid also has additives for
corrosion protection which, over time, loose effectivness
(and you don't want expensive ABS and brake parts corroding).
I have the fluid flushed when I replace pads (which is around
every 30K but with a clutch your pads probably last longer since
I bet you brake less).

Castrol/Girling are Europes largest manufacturer of Oils and fluids, the one you have chosen is perfect for what you want, DOT 4. Be very careful to keep the container tightly sealed at all times, Brake fluid is hygroscopic (absorbs water) and containers must be kept air tight. The electronics you see in the cap are a part of the fluid level indicators and will not come to any harm.
If your system is losing fluid it might be a good idea to have it checked by your Volvo shop, you should certainly not have to add more than a teaspoon or so as part of routine maintenance.
John Scales (1966 850R)