Consider the following words as examples:
1) Version 2) Progression 3) Differentiation 4) Fission 5) Fusion 6) Insurance 7) Sure
I pronounce all these words with a 'sh' sound even though it is not explicit in written. I have heard people using some of these words with a 's' sound, especially #6. Both myself and the folks I have heard talking are non-native English speakers.
Are there any defined rules that tell when 's' or 'sh' sounds should be used?