Originally Posted By: LeakySeals
I acquired a free 03 Maxima with a 3.5L with the infamous busted radiator/tie bar support. When I got it the 3.5 was dangerously low on oil. I added 3.5 qts! There are a couple of leaks. One is the cam sensor o ring. Another is from the pan area. I assumed its the drain bolt washer. Could that possibly the o ring you describe??
The O-ring is behind the "oil cooler"......the round thingy that the oil filter screws onto. You remove the filter, then there's a big nut behind it (if memory serves correctly). The housing (oil cooler) is then able to move away from the block. There is a big O-ring behind it where it mates to the block. They tend to start leaking above 100k miles. Get the replacement from the dealer. They've been redesigned over the years and I think I only paid about $8 for it.
Edit: There are two rubber hoses that connect to the oil cooler. They contain coolant. Be careful with them..........unless you're replacing them also. Mine were oil saturated from the leaking O-ring, so I replaced mine.