Can you put R30 in a 2x8 wall?

Yes, you can put R30 in a 2x8 wall, but whether you get a true R30 depends on the type of insulation you use. A 2x8 wall cavity is 7.25 inches deep. A standard R30 fiberglass batt is typically 8.25 to 9.5 inches thick and is designed to fit a 2x10 or attic application, so if you put that batt into a 2x8 cavity it will be slightly compressed, but compression of less than an inch is acceptable and will still give you close to R-28 to R-29, which is generally considered acceptable by most builders. Better options for a 2x8 wall are to use insulation specifically designed for that depth. For example, 7.25 inches of closed-cell spray foam at R-6.5 per inch would give you R-47, which is far above R30, or 7.25 inches of dense-pack cellulose at R-3.5 per inch would give you about R-25. You could also use two layers of batts, such as R-19 plus R-11 to fill 7.25 inches without excessive compression. So yes, a 2x8 wall is actually ideal for achieving R30 and even higher, unlike a 2x4 wall where R30 would be severely compressed and lose performance, a 2x8 has plenty of space for R30 to perform properly.