Whether you are trying to rebalance an existing fluid or creating your own fluid it is important to know how the density of the lamp works. The short explanation is that as the wax heats up and melts it expands and it's density lowers. The density of paraffin wax is very close to the density of water. In Lava Lamps paraffin wax is mixed with other chemicals to make it just slightly more dense than the liquid it is in when cool. As it heats up it expands and becomes less dense and begins to float. When it reaches the top it contracts as it cools down and becomes more dense and sinks.
To make a Lava Lamp flow you need to find the sweet spot for this change in density to cause flow. This can be done by adding denser fluids which allows the wax to float. You only need to add a small amount to your liquid mixture. This depends on if you are starting from scratch or using an existing liquid.
If you are starting from scratch it is important to start with either distilled or reverse osmosis water. That is because tap water can have high contents of minerals and other contaminants that will alter the density of the water and potentially introduce chemicals that will shorten the life of your Lava Lamp.
Propylene Glycol can be easily found online. In the USA it can be found at Tractor Supply stores. Beyond adjusting densities in Lava Lamps propylene glycol has many other uses so if you are having trouble finding some look for suppliers of industries that use it.
Glycerol also known as "Glycerin" can be found online.
You will also need to add a surfactant to the liquid. This should be done before turning the lamp on since without this the wax will stick to the glass. Two surfactants that are currently known to be suitable for lava lamps are sodium lauryl sulfate and Triton X-100. The more surfactant you add the higher chance you will have for the liquid to be cloudy. Anything over 4% will definitely cloud the liquid so using a mixture of the two surfactants allows you to add smaller amounts of each to reduce the risk of clouding
Start with a base mixture that is slightly less dense than the original fluid. A good base to start with when working with Lava Lite lamps from the 90's is as follows.
0.01% Triton X-100 (1ML)
If working with modern lava lamps skip the propylene glycol and glycerol altogether and use pure water with surfactants and modify the density with the steps below.
To alter the density add glycerol in small increments of a few milliliters of glycerol at a time while the lamp is fully heated. This can be done with a dropper, syringe, or any other tool that allows for measurement down the the milliliter level. Once the wax starts to flow watch how it performs and if it runs for a normal 8 hour cycle you are done. If it overheats you need to reduce density by adding water.