as far as the difference between the ACCT and the MCCT, the ACCT is the automatic one (stock from factory, and it will eventually fail, there is a spring in it that dies). the MCCT is a manual one, you basically adjust it when its installed, then forget about it for thousands of miles when it may need adjusting again..
oh, btw.. ACCT=Automatic Cam Chain Tensioner, MCCT=Manual Cam Chain Tensioner.
work on the ACCT/MCCT is easy, and does not need any engine pull or even major parts pulled from it. they are on the back side of each cylinder, the front one is easy to see, rear one a little tougher (its kinda hidden by the coolant bottle). 2 bolts each to remove/replace
that o-ring you need is a pretty common problem, if you need to seperate that joint for any reason, and sometimes just movement there will cause it to leak.
when you go to replace it, make sure to clean the surfaces (both the part the o ring goes on, and the housing itself), good. else it will leak again. been there, done that, even with a new o ring. my 2005 had a some deposits and small amount of corrosion in that area, so I can probably guess yours does too.
I got my o ring out of an assortment I got from the local autobone store.