Your recommendation is to use air con regularly to avoid dry seals but my car manual recommends leaving air con on to enable the climate control system to function but later on the quick start manual it advises that using the air con can increase fuel consumption by 10 - 25%. This seems a significant amount but with a huge variation in the increased consumption - which is likely to be the more accurate figure? In the past I have always left aircon permanently on to avoid the problems with seals drying out in my Peugeot 406 diesel which returned a steady 41mpg. However, with my Toyota I'm getting near 33mpg so a 25% improvement in consumption would be welcome.