This was a known problem when the 335i was initially launched. N54 runs real hot & even with the factory oil cooler, some people get their temps up real high (people who live in "hot" states & people who put their cars on a track/drag strip). I'm in NJ & usually average 240-250, which is fine for right now but even I'm a little concerned about aggressive driving in the middle of July. An oil cooler isn't a priority, but I'm planning on eventually going with this one (after I decide what body kit I wanna go with)
http://www.topgearsolutions.com/evraspoilcof.html