However, "Petroleum Naptha" is how it is referred to by
the Canadean Environment ministries, the Ontario Energy Board, and
Imperial oil.

That may be, but that is a slovenly corruption of the spelling, which
follows a natural mispronunciation. Kind of like people say

op-tha-mologist (incorrect but easy)

instead of

oph-thal-mologist (correct but clumsy)

And thus

nap-tha (incorrect but easier)

instead of

naf-tha (correct but not as easy)

See _The American Heritage® Book of English Usage_, 1996:

... with diphtheria, diphthong, and naphtha, the ph should properly be
pronounced (f), not (p). Because so many people say (p-), as opposed
to (f-), though, this variant has gradually become acceptable.

(I dedicate this post to the memory of the late Wm F Buckley.)