I have been using Squirt for the last 4 years and think that it works really well and is my preferred lube. I find that the drive train seems noticeably smoother and quieter when I use it.
Initially I used it as a 2-3 season lube, changing to a wet lube for the 5-6 months over the Winter. This was because, following a few wet rides, when I first started using Squirt, and when I didn’t immediately clean and dry the bike I noticed some rust on the chain and cassette.
This year, more by accident than design (we‘ve had a reasonable dry winter and so I never quite got around to cleaning all the wax off the chain and applying a wet lube), I have run Squirt through the winter and so far so good. It is a water based lube so will wash off eventually in wet conditions, but I find that as long as I’m reasonably careful it works well…..for my riding and location. For example, I almost never ride for more than 3 hours; I live/ride in an area where the majority of the trails dry and drain well; I have the flexibility to be able to arrange my rides around the weather so I rarely ride when it’s actually raining or immediately after a downpour; In the winter I wash the bike and chain after every ride, dry the bike and thoroughly dry the drive train including final dry of the chain using very absorbent kitchen roll…….I have a friend who runs Squirt all year round and is even more OCD than me and uses an old hair dryer! I then apply the Squirt to the chain immediately after cleaning so it has plenty of time to dry before the bike is used again.
So while it has worked for me this winter, if I was doing longer rides, in wetter conditions and not always having option/being bothered to clean and dry the bike after every ride then I’d probably, reluctantly, go back to a wet lube during the winter months.
One option if you have a couple of bikes might be to set one of the bikes up as your winter/mud bike with wet lube and use that bike of the wettest days and carry on running Squirt year round on your main bike.