The engine oil cap on an engine 2JZ-GE (Lexus GS300 1995) says to use 5W-30. The general search for the wet compression says to squirt about one teaspoon of 30-weight motor oil into the spark plug hole.
What oil should I use specifically for wet compression? Can I use 10W-30? Are there other (cheaper) fluids to use for the purpose?
You can use what the engine was designed for it doesn't really matter it's just oil will fill gaps of the piston rings are worn so the pressure would go up a bunch showing you the piston rings are worn
Use any engine oil you have.
I have 5W-20 only, whereas my engine is designed to work with 5W-30. Can I use 5W-20 for the wet compression test of one cylinder (out of 6)?
You can use ANY oil you have around for a compression test. All it does is temporarily seats the rings.