When I bought my bowser, it was about half full of avgas which had been delivered to it about 12 months before. I discussed this with the fuel supplier who suggested that it was OK to use but it would be a good idea to top up the tanks with fresh avgas on a 50/50 basis. This I did and had no problems. I now find that I fill it about twice a year, so on average the contents are less than 6 months old. Look up the previous thread to see details of the experiments that Gaseous did a couple of years ago. Mine was made by a company in Birmingham called Trailer Engineering. They are still there and are very helpful.