The twin-bulb 850 headlamps don't deteriorate as fast/badly as the later S70-V70 lamps (with short-lived reflectors), but they aren't spring chickens, either (they're 22 years old, assuming originals). The lenses are glass, which means they won't be hazed but very likely could be pitted from years of road grit. Even pitting that "just looks minor" does a really bad number on the headlight performance: less seeing, more glare, more backscatter. And the reflectors aren't immune to age, either. By the time you can see degradation enough to notice, they're well past needing replacement.
And speaking of replacement, aside from the usual caution (don't use aftermarket lamps if you can possibly avoid it; they're junk whether they're designed to look just like the originals or otherwise)), be aware that contrary to what some Volvo parts vendors claim, these headlamps are not designed to have their lenses replaced, and doing so usually ruins the lamps permanently. Some of those vendors go further and claim you can convert your US lamps into European ("E-code") lamps by swapping lenses. That's not true, don't fall for it.
As far as bulbs go:
Vosla HIR2+30 in the low beams and
Toshiba HIR1 in the high beams, trimmed to fit as per
here, and then the lamps aimed carefully and correctly with an optical aiming machine (not just shining on a wall or board).
For signal lamp upgrades, use
Stanley 3496 in place of P21/5W or 1157 (dual filament) and
Stanley 3497 in place of P21W or 1156 (single filament). No amber version of either bulb is available. Use a good brand of
2357NA in the front turn signals. The PY21W rear turn signal bulbs tend to lose their amber coating; fix that by using
Diadem bulbs (Osram ones specifically).
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