Can I USE PC3-12800 DDR3L DIMMs in a MacBook5,1?

No. Do not install memory that is not the same spec as that which is specified by Apple. Buy the right memory product for the computer which is 204-pin PC3-8500 (1066 MHz) DDR3 SO-DIMM. Testing by OWC indicates you can install 8GBs of RAM - two 4GB modules.

DavidAMcL wrote:

Is this likely to work? Would there be any risk in trying the newer, faster RAM in the older MacBook5,1?

I wouldn't say there's a risk in trying as long as you install it correctly. But in general Apple notebook computers are known for rejecting RAM that's spec'ed faster than the factory specification. If it's just a salvaged part from another computer, knock yourself out and let us know how it worked out. If it doesn't work you'll just get the three beeps.

I've tried all sorts of things with my 2007 (Santa Rosa) MacBook. I ended up putting in PC2-4200 modules from other computer into my machine that spec'ed PC2-5300. It actually ran properly although I know it was slower. I also mixed and matched with the factory modules or aftermarket PC2-5300 modules with a single PC2-4200 modules and it would always operate all modules at the same speed as the slowest in the pair.

Another thing you might try if it rejects it at startup is to mix and match with a slower module. For some reason that seems to work sometimes if there's a slower module that meets the spec.