if you have tpms then you have to use a cap compatible with tpms, although a regular plastic cap with no metal could also do the job. do not use just any old metal cap though, it will seize on there & ruin the tpms sensor... fas as mixing nitro with regular air, you can do that no problem but as the nitrogen is diluted with regular compressed air so is the benefit of nitrogen removed. benefits of nitrogen is #1 it won't absorb moisture & #2 the molecules are larger so you will not have any normal air seepage as you would with regular compressed air... pressures are more stable with nitrogen