High viscos(thick) lubrication will prevent the ball bearings from circulating freely. If it's too thin you'll need to re-apply often.

I recommend Super Lube 51010. It's an excellent PTFE-based lubricant. 51014, 51004, 51008, 51025 can be used as a substitute. They're the same product but in a different packaging.

Shake the bottle well. Apply 1-2 drops into the ball bearings on both sides of the carriage block.

If you notice dust and hair collected onto the rubber seal give it a quick wipe with a microfiber cloth. Don't forget to do the opposite side of the block.