Capacitor leakage and design for low energy battery powered devices
Ceramics are usually the first choice for capacitors.  One of their several advantages is very low leakage.  The only reason not to use ceramics is if you need so much capacitance that they would take up too much space or cost too much.

At 16-20 V, you can get 10s of &micro;F from ceramics.  If that's enough, there is no reason to look further.  If not enough, first try paralleling a few, then try specifically designing the circuit to require less capacitance.  If none of those work, then you probably need to use electrolytics and live with the higher leakage current.

Note that leakage is probably specified at the worst condition, which will be highest voltage and highest temperature.  Maybe the datasheet will tell you what leakage is at lower temperature, but you may have to contact a application engineer from the company to get definative specs.  That's probably a good idea anyway, as they may be able to suggest other things you can do to minimize leakage.

Again though, the first reaction should be to solve the problem with ceramic capacitors.