Testing bullet lubricants can be a pain if you want to fill the grooves. Filling a lube-sizer is really prohibitive, melting out the old lube, perhaps cleaning with solvent, loading with the new lube is a LOT of work and takes a LOT of lube. This is something I only do if I am satisfied the lube is going to be used on thousands of bullets.
There are several methods of filling the lube grooves without a lube-sizer. Pan lubing works OK, but you need a lot of lube to fill the pan. You can dip the bullets into hot lube, coat the bullet then use a fired case to remove the excess. If you use a small container, dip lubing uses a small amount of lube.
Amount is important if you want to experiment with different percentages of components. Even a 2 component lube tested at a spacing of 5% is 20 different lubes. I don't even want to think about some of the 3, 4, or 5 component mixes currently popular.
Both pan lubing and dip lubing require melting the lube. Some lubes have very high melting temperatures, and some components can decompose or change if they are melted. Syringe lubing only requires softening the lube, this is usually possible at modest temperatures.
Your local hardware store may have plastic glue syringes, if not try a hobby shop. The hobby store or hardware store will have brass tubing in fractional sizes. Get a size which will fit the syringe tip and a size a little larger than the bullet. If your cartridge has a long enough neck and the bullets will fit in a fired case, you can cut off a cartridge neck and use that. Wrap 400 grit sandpaper around the large tube and sand the small tube to match the contour. Cut the tubes to appropriate length, you want the large tube to be a ring which will cover about twice the width of the largest lube groove. Solder the small tube to the side of the large tube. Using a drill inside the small tube, drill thru the large tube. Use the sandpaper on a dowel to remove the flash inside the large tube. The plastic syringe may have a small nozzle opening, it will work more easily if you drill out the nozzle as large as possible None of this takes more than simple hand tools.
If you are testing un-sized bullets you are ready to go. If you need to size the bullets you can run them through a Lee sizer or your lube sizer. Coat the bullets with a case sizing lube, size them, and then wash off the lubricant.
If I am testing gas check bullets I seat the gas checks in my SAECO lube sizer first, it seats the check aligned and fully on the shank. If you use the Lee size die, the gas check needs to be fully seated before sizing, tapping the bullet on a hard metal surface will usually fully seat the gas check.
You can use very soft lubes at room temperature, but even soft lubes are easier to use warm. I use a small insulated plastic cooler, fill a quart jar with hot water, and put it in the cooler. Equilibrate the lube, syringe and bullets in the cooler, warming the bullets makes lubing easier. Equilibration will take 1 to 2 hours, be patient. You may have to refill the jar with hot water. You want the lube soft but not melted, about like room temperature Crisco. Adjust the water temperature to get the temperature you need. A thermometer in the cooler is useful, 90 - 120F seems to work with most lubes. Pull out the plunger, fill the syringe from the back. I use a small spatula to fill the syringe. Put the plunger back in, you are ready to lube.
Push the bullet into the tube, put pressure on the syringe, and fill each groove you want filled. In 10 minutes you can lube 50 bullets. With multiple syringes, you can do 500 bullets with 10 test lubes in 2 hours.