I always drain the oil cooler. I think it takes a 27mm wrench but a Vise Grip will do in a pinch.

I don't take the oil line fitting off all the way, I just loosen it and let it drain overnight along with the oil pan plug removed. That way there is no chance of cross threading the oil line nut when tightening it.

I've been doing this for more than 10 years and never had any problems. I like to get as much of the old oil out a possible.

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