try to get inflatable boot trees instead, they're shaped more like legs and will keep them upright without stretching your boots in a weird shape. I use a beeswax product instead of oils, it keeps them soft and repels water to keep any cracking to a minimum
Any boots made of nice leather will stretch. You can use mink oil more than once a year. This will keep them more supple and soft. you can use other leather balms and conditioners more frequently. Boot trees are also very good for maintaining shape, but you have to make sure that they aren't too big for your boots so they don't stretch them unnaturally, altering their fit on your legs.