Buy the exact same RAM if you can.

Yes, you should absolutely get another 8GB stick of the same model if possible. Mixing sticks is usually fine, but the safest, most reliable path is identical DIMMs.

If you can’t find the exact same model, get another 8GB stick with the same specs: speed (e.g., DDR4-3200), CAS latency (e.g., CL16), and voltage (usually 1.35V). The brand matters less than these numbers, but different manufacturers can still cause weirdness like not running at rated speeds or occasional crashes. Your motherboard will likely downclock both sticks to the slowest common denominator anyway.

If you’re already having trouble finding a match, consider buying a 2x8GB kit and selling your single stick. It’s a few extra bucks but guarantees compatibility and dual-channel performance. Single-channel RAM is leaving performance on the table, so don’t delay.

Either way, you want dual-channel mode. That single stick is bottlenecking your system.