I think that 20 years for a unknown brand is too long.
Do you know the "stolen formula" story ? https://en.wikipedia.org/wiki/Capacitor_plague
That means the sooner you get rid of those capacitors, the better. Capacitors from before that era might still be okay.
The price for a new high quality capacitor is low, using the old ones is not worth the risk.
At Reichelt, you can buy capacitors from a good brand, for example Panasonic, Chemi-con.
Look for the 105°C, 5000-10000h, low ESR, capacitors.
My parts bin consists in part of components I was gifted by my dad. The date back to the period of roughly 1960 ~ 1985. The electrolytics in the bunch are most likely from the later part of that period. Most of them are made by Philips. I've used several for testing and also in assembled projects. Zero failures so far, also after extended periods of use. I never bothered to reform them after their decades-long dormant period.
I have a lot of Soviet capacitors from the 80s for 1000, 2000, 4700 and 10000 uF and almost all of them work fine. Of course, I check their capacitance and leakage current before using.
The only drawback is the size.
The discussions online these days about capacitors, in general, borders on uninformed hysteria.
If the value is within tolerance and the ESR is reasonable, there is no reason not to use them in a hobby project. My exceptions would be electrolytics stored at high temperatures, which will of course dry out and lose capacity or anything over 50-100V since things can get violent should they rupture.
I routinely read about people ripping perfectly fine electrolytics out of otherwise inoperable gear without one seconds worth of investigation. Given their experience level, you know it will never be repaired because they will undoubtedly do more damage removing the old caps than had they left things as-is and spent some time doing a good visual inspection and then actually troubleshooting.
From my (limited) experience most of aluminum caps salvaged from old electronics are still within specifications. My testing options are limited (leakage by DMM, capacity and ESR with the ATMega based LCR tester from eBay; all at room temperature only) and they are usually from low power devices.
In fact I have encountered damaged electrolytics only once. It was a power source of a LCD display: high ESR, capacitance about 10% of rated.
I presume you mean goop leaking out of the can... Current leakage under working voltage is also important.
For low voltage I'd just use them - large high voltage caps are another matter since the caps may need reforming carefully to avoid internal damage - reforming is just gradually rising the voltage while monitoring the leakage current - leakage current should fall to within specification before stepping up the voltage, rinse and repeat till at working voltage. This regrows/refurbishes the oxide layer on the anode that forms the dielectric of the cap.
With low voltage caps there's not enough energy available to cause damage, they will reform on first use (the leakage will drop in the first few seconds/minutes).
Having an ESR/leakage capacitance meter would be ideal of course, but you'll likely seldom need it again...