skinny bottle brushes?

Shake a mix of soapy water, ammonia and acquarium gravel (or BBs, or other small objects) in the bottles to scrub them.

I soak in hot water and Dawn detergent, rinse and fill with hot water and drop in a denture tab. If that still doesn't work, hot water and dishwashing powder.

You can also put a handful of rice or sand along with a bit of dish soap and a little hot water in the cruet, and give it a good shake. Works for me.

In a large bowl or saucepan or whatever, mix some dishwasher detergent (like Cascade) with very hot water. Let the cruets soak in this solution for an hour or so, then empty the cruet, refill about halfway with the solution, stop up the opening with a finger and shake vigorously. Repeat the soak/shake cycle if necessary.

Even better is to use a detergent that contains enzymes (like Cascade Complete).

These soaks have worked wonders for me on all sorts of stuck-on, burned-on, and other accumulated crud.

Try using a pipe cleaner, it bends to any shape you need. I use them frequently for cleaning my hummingbird feeders. Dawn dishwashing liquid is great on oil and grease, and hot water is much better for dissolving the oil. Good luck

If you have cut glass...and/or lead glass, you will scratch it with gravel or sand or BB's....and may likely crack it with very hot water.....
Moderatly hot water....mild soap....a long soak....pipe cleaners if you are very careful to keep the metal core off the glass will be the safest thing to use.
Old glass ( pre 1900) is quite fragile and easily scratched....newer glass is sturdier....unless it's lead glass.
Linda c

Easy-off oven cleaner.

Take a piece of cloth, maybe 2x2 and put inside with watm soapy water, give it a good shake, and the cloth will bounce around the inside and clean it , use tweezers to get cloth out, probably soak it for a while to soften up the grime

Baking soda and vinegar, maybe.