Uh.....what planes are you on where you, the passenger, can simply "pull down the oxygen mask"? Also, wouldn't the P95 only help with particulates (e.g. soot), but not with the actual toxic fumes?
3M 8577 has a bonus carbon layer for this purpose. Its protection is not complete, but can limit the damage. You should also carry spares in case the carbon layer is exhausted.
Also, you start to smell the diesel fumes. The more you wear a clean mask, the more sensitive your nose becomes to mild fumes.