What happens if I install slower RAM than specified compatible with the motherboard?

Specifically, will there be any instability or other problems if I install DDR3 1066MHz (PC3-8500) RAM with a motherboard rated to be compatible with DDR3 1600MHz (PC3-12800) & DDR3 1866MHz (PC3-14900), using Crucial's system scanner.

The machine came with a single 2GB 1600MHz stick. I had 2x 2GB 1066MHz sticks lying around, so I was wondering whether I could use them.

I tried searching, and only found answers for the other way round (RAM faster than motherboard specification).

Solution 1:

It is impossible to predict what will happen.
Some motherboards run fine with slower RAM (just a bit slower), others will have issues.

But you can't damage the computer by trying.

Just install the strips and then run memtest86+ for at least half an hour. If it comes through without showing any errors it should work fine.

If memtest fails it's not going to work in the normal OS either.