When the oil in your automobile or boat motor degrades, you change it. You should do the same with your reels to keep them in good working order and lengthen their life span. Several times in the past I've learned a hard lesson. I've had reels seize from lack of cleaning. Don't let this happen to you.
How do you know when to replace the lubricants? Take a look at them once in a while. You'll most likely have to open the reel to look at the parts. If the oil is not clear or has obvious debris in it, it's time for a change. It usually will be some shade of gray instead of the translucent golden color of fresh oil. The gray color is very fine metal particles and any grit that got in there. Greased parts can go longer without cleaning, but it's convenient to do it while the reel is apart.
The list of things you'll need is pretty small. Some screwdrivers and a pair of pliers is about all you'll need for tools. Paper towel, swabs, and a toothbrush dipped in denatured alcohol will remove old lubricants. Some new oil and grease to replace the old stuff. I bought some reel oil and grease a long time ago. It will last my lifetime. You could also use the same stuff you use for your car.
When you take apart the reel, lay the parts out in the order that you took them off. This way you will know the order they need to back together. Try to clean up all the old lubricants. The paper towel is good for wiping any surfaces, the swabs are good for tight spots, and the toothbrush works well for cleaning the teeth on gears. Pay attention to the type of lubricants you are removing and where they go, so you know where to put the new stuff.
When the reel is back together you will be pleased with how smoothly it operates. Especially if it was quite dirty. You'll also have the peace of mind that your reel will not seize when your on the water.