I think the isopropanol in the various brands of gasoline antifreeze and drying additives is more water free than the 91% from the drugstore, at least until you open the bottle. It is also available as methanol. Look on the label to see which it is. I have a batch of ascorbic acid and phenidone stock in methanol from the auto parts store that is still OK after several weeks in a 100 ml bottle with more than half gone. There was no real heating needed for solution, only a mild warming. From what you say about the rate at which you use developer, this might be a good thing.