this is taken from a rotary thread :
as long as the substitutes you use are viton or nitrile ( in some cases )
then you are using two rubber compounds that are very resistant to oil and most fuels
and are also more stable than most with temperature
- nitrile and viton both have an Achilles heal
viton is not as stable as liked in presence of aromatic based solvents ( specifically ketones )
nitrile is not as stable in heat as viton , and doesn't like ester acids
and when formulated to be resistant, can loose its flexibility
( at any rate the two remain the better of the resistant rubbers )
the other O rings used by mazda throughout the motor appear conventional butyl nitrile
of which the viton could actually be considered a superior upgrade
( though most O rings sold in kits are likely nitrile )
viton is , by definition , a flouro- rubber
O-rings made of viton are typically color coded as black
its 200 c stable
and proves to be more flexible than nitrile when specifically formulated to be heavily solvent resistant
mazda choose to flourosilicon coat on the early green type oil control ring
( the simple cheaper black one appears nitrile, but may be possibly be viton , but is not coated )
mazda have used the flourosilicon coat onto the nitrile
to provide a protection for the rubber for its known weakness