I just bought Castrol dot 4 to change my brake fluid. Last time I used newly purchase Comma fluid and it has been fine. However, I noticed that while comma just seal the fluid with a normal screw cap, the Castrol fluid had a foil seal under the cap that had to be broken, so I would trust that more if I kept it for a year or two. By the way, never use Carplan brake fluid, I had some that I had bought on holiday only a month before, and used it to change my clutch fluid. After a few weeks, I removed the cap to check the level and found crystals all over the cap and the fluid looking very cloudy. I changed it immediately with some Comma fluid and I find it looks much better after a year than the Carplan did after weeks, and have never had the crystals again. Duncan