hmm... 90k service DCT oil while the capacity just 4L? My ford with wet clutch has a capacity of 6L with factory recommended interval @ every 60k, but for me i do it every 40k km.

Probably cartridge type filter

My wet clutch already served me over 200k km and never a prob, i change the fluid at every 40k instead of 60k, does that help, i say probably it did.

Well... depends on the design, on some toyota it will need a special tool to open it... so if your outside mechanic don't have the special tool to open it and find something to whack it to open high chance it will cause damage, if damage then you are talking bout changing the whole housing. If like mercedes which is on the top and don't need special tool to open while the housing is made of metal, that means easier to swap out the oil filter, meaning that you can even do it at home if your maintenance regime calls for changing the filter in between oil change interval, cause it won't spill cause it is on top when you take out the filter. The one on my Ford is also on top, don't need special tool to open, but the housing is plastic, i m pretty sure if open too many time will broke the plastic housing someday.