tldr; Use Lamp Oil. Nothing else.
For 5 years I was Lamp Man at my local station on the local heritage railway, deploying 13 lamps around the station from ground mounted shunt dummies to some rather challenging heights on the Inner Homes.
All was fine for 2 years, then, a couple of weeks after we received a new delivery we found that the lamps were fading and sometimes extinguishing, even with plenty of oil in the reservoir. Inspection showed the tip of the wick was completly sooted up, preventing oil from making the last part of its journey from reservoir to flame. For a few months I had to trim the top 10mm of wick every time I put the lamps in. Eventually we gave up, donated the rest of the tank of 'lamp oil' to the engine shed for loco cleaning and got a new delivery of proper lamp oil. Problem solved.
I never found out definately what had gone wrong. My observation was that the faultly batch either had disolved impurities that were left behind when the oil evaporated, or their was a poorer burning oil mixed in that left behind a sooty deposit when burned. I do know that the railway had to change supplier because the old one had stopped selling it due to low demand. And when we did get our delivery of proper lamp oil we had to wait for other customers in the area to order to make it worthwhile sending a tanker down from the midlands.
Proper lamp oil will smell like paraffin, and will self evaporate if spilled. I was told it was a more refined grade of paraffin with fewer impurities.
My understandng is that signal lamp wicks and reservoirs are dimensioned to run for a week so that refilling and re-lighting was a weekly task. When I tried this I found it to be a challenge, I could only reliably achieve 5 days. The bulls eye lens on a signal lamp is very effective, a very small flame (4 mm?) can be seen at a remarkable distance. We also possessed but did not use a couple of lamps that had a bi-metalic strip that drove an electrical contact intended to be wired to a warning indicator in the signal box to show if a lamp was out. (All semaphore signals at the station were visible from the signalbox, so at night the signalman could either observe the aspect directly, or see the backlight/blinder).