Alternative RAM modules compatibilty

Using slower OS-DIMMs while possible will hold your system back. I strongly recommend sticking with what Apple recommends. The cost difference is not enough to fight over. Many people often get the next higher speed SO-DIMM’s if they can’t get the correct SO-DIMM’s.

Mixing different speed OS-DIMMs can make a system unstable. While the bus clock will be at the slower speed, there is other aspects of the RAM that are not visible which can cause issues. Here’s a deeper look Memory (RAM) Timings & Latency: CAS, RAS, tCL, tRCD, tRP, tRAS

Check your RAM configuration by opening: About This Mac > System Report > Memory Your RAM should be running all at the same speed bus speed.