I deal with o-rings all the time at work, we use viton a lot because of the temperatures of our products. That said i would recommend using NBR if you can get away with it as viton is pretty soft and damages easier especially is something is disengaging and re-engaging like a valve seat, also deforms quicker over time. They are good to around 200C (~390F), but do drop in duribility over about 120C (~250F). From memory NBR is good to about 120C (~250F), if your intake or BOV is getting up to those temps you really need to look at reducing it somehow, heat shields and wrap or something. you could try and get a higher duro NBR o-ring, will be tougher.