In my ThinkPads, I have mostly used Corsair, Crucial and Kingston over the years, and found they all seem to work well. More recently, I have started using Geil and Patriot with newer systems (W510, X120e, X220) and they work fine as well.
As long as you get memory which matches the electrical characteristics and timing of the existing memory, you should be fine. If you are looking at purchasing a "genuine" stick from Lenovo to get a matched pair, I would suggest looking at replacing both
SO-DIMMS with third-party memory as this is often cheaper than purchasing a single SO-DIMM from Lenovo. This is what I have done, especially when companies like NewEgg have holiday deals and I can max out the RAM in a system quite inexpensively.
One thing you might want to check are the unofficial
specs for your computer's RAM at a place like ThinkWiki
, the ThinkPad user's mailing list or using Crucial's memory inspector. It may be that your model can support a larger amount of memory like 16GB (2×8GB). Of course, Lenovo doesn't officially support some of these larger amounts of RAM in some systems (usually because the larger SO-DIMMs were not available when they were qualifying memory for the system) but they tend to work fine. Keep your old SO-DIMM handy, though, should you ever need to send the machine in for service.