this photo is of a replacement motor unit that is said to fit volvo 240, 740 models, as well as some SAAB models too
If this is the sort that is found in most 240 models then its one that has guts virtually identical to the ones in a BMW I have owned. Unsurprisingly after twenty or thirty years the grease inside the gearbox does dry out! I used some grease in which the lubricant is a synthetic oil and the grease also contains PTFE; my experience is that this grease doesn't dry out or change colour even after twenty years or so. Probably there are other greases that have similar properties.
BTW these motor units usually contain a screw adjuster in the housing which is in line with the motor shaft. This adjuster controls/adjusts the end float in the 'worm' gearbox shaft.
Since there seem to be many Bosch motor/gearbox units which differ only in detail, I think it is often possible to (say) swap motors between units which are found in quite disparate cars; there may be minor internal differences between (say) a volvo and BMW motor/gearbox, and differences in the gearbox housing and wiring connectors. Probably other makes of car us similar parts too. Thus quite a lot of swapping about should be possible.
cheers