Does any body know where I can get a larger aftermarket oil cooler for my 400ex?

 

I would be interested in seeing what that actual oil temperatures are. I had a 1998 XR250R with no oil cooler at all. We would swap a dip stick oil temp guage back and forth between several XRs to compare oil temps. The 250 would often be at 270 deg while the liquid cooled 650R was at 220 deg and the XR400 was at 240 deg. All bikes were stock except for slight unplugging and rejetting. The XR400 has a small steering stem mounted oil cooler, much smaller than the big stock 400EX oil cooler. The hot oil temps did not seem to bother the XR250. My family rode this bike for several years and then sold it to others in my MC club and bike is still running very well and very hot. So I think you should check oil temps and if not above 270-290 then you may be OK as is. I would rec'm a good synthetic motor oil such as Shell Rotella T full synthetic 5-40 or some motorcycle specific full syn if you want to spend the dough and then change oil every couple hundred miles max. You should also be very carefull that your jetting is not too lean and that you are running high enough octane gasoline.