I just bought a 55 Mercury Monterey and this is my first experience with a Y-block.  I purchased an oil filter conversion kit.  I assume that the canister filter is the large cylinder at the back of the block on the drivers side.  Also, on the drivers side to the front of the block, below the head and behind the fuel pump is another canister that is smaller and has a vacuum feed from the carb.  Can anyone tell me what this is? 

Also, when I change the oil what weight do you recomend.  I am in Utah so it will be very hot here soon.

Thanks