Comment by Melatonic

If you're using Windows on it I highly recommend looking into the last version of Windows 10 LTSC - it's basically a completely stripped down version without all the crap (no spyware, windows store, candy crush, etc). Anything you really need to add back is easy (like if you do need a windows store app for example)

With an SSD it boots insanely fast and consumes very little resources at idle. And will get security updates for many years to come. It's really the perfect OS for the T420 (or Linux of course - although I'd actually prefer Windows for a classic Thinkpad).

Bit hard to buy legally (you need to go through a VAR - there are some who will definitely sell you single copies). Core Windows 10 without all the bloat is actually a really impressive and lightweight OS considering the massive backwards compatibility and huge backlog of free software. Not to mention the insane breadth of hardware it will run on.