Assimilation

<<What is your dialect Lazar?>>

Eastern New England, with strong General American influence.

<<I guess that British tend to use this assimilation, probably RP:
/θ/ changes to / s / before / s />>

No, I don't think so.

<</ z / changes to /ʒ/ before /ʃ/ or / j />>
<</ s / changes to /ʃ/ before / j />>

I believe those two are pretty common, but being followed by a rounded vowel is irrelevant. I would correct them this way:

/ z / and the following sound changes to /ʒ/ before /ʃ/ or /ʒ/ or / j /
/ s / and the following sound changes to /ʃ/ before /ʃ/ or /ʒ/ or / j /

I think it is interesting to notice that some assimilation occurs even if the second word is stressed, while other kind of assimilation doesn't seem to behave this way. Example:

For the second time this year, etc. --> thish ear ("ear" is stressed)
I don't care about him, I care about YOU! --> abou' you! (no assimilation, a glottal stop is used instead of saying "abou choo" and stressing "choo". That would sound weird to me)

That's weird, LOL. What do you think?

Isn't the assimilated "this year" pronounced [DI.s\ji@`], with an alveolo-palatal fricative ([s\]) and retaining the [j]?

To me the /sj/ part sounds exactly like my "ch" in the German word "Märchen" ([mE6.s\j@n]).

Off topic: The "ich"-Laut /C/ becomes [s\] in my dialect, not [s\j]. It's just that the diminutive suffix "chen" is pronounced /Cj@n/. "Märchen" does not rhyme with "pferchen", which is pronounced [(p)fE6.s\n=]. Just in case you wonder...

http://en.wikipedia.org/wiki/X-sampa