The capacitor rating is very important, but the brand isn't. If yours uses a 40uF capacitor, then you must use 40uF. If it uses a different capacitance than 40uF, don't use your spare. The manual should state the capacitor spec, or just look at the one on the genset.
I suppose there could be quality differences among brands, but I've never encountered it within a genset application. We would use unbranded $8 ones sourced from an HVAC street vendor in Colombia that worked and lasted the same as the $125 ones the genset manufacturer sold. Mostly, I'd just buy $10 ones off Amazon.
You also need to make sure the voltage rating is above your use voltage, but if you have a 120V genset, all of them are above that.
Capacitors usually don't fail by lowering the voltage. Well, they do, but it is usually a short one-way street down to 0V. If yours has been running for a while at 100V, that is a bit odd. But it is very cheap and easy to throw in a new capacitor to determine that.
When changing it, make sure the capacitor is discharged before touching the terminals. Stick a screwdriver across the terminals first. If your spare has multiple prongs on each terminal, it doesn't matter which prong you use. If your spare has three terminals instead of two, then it is a start/run capacitor. One of the terminals will be labelled "fan" or "start". Don't use that terminal. Connect the leads to the other two terminals - doesn't matter which lead goes to which terminal.
Mark
