The stock FC3S oil cooler is good unit for a stock OEM oil cooler.  It is one of the largest OEM oil coolers ever made for a mass production automobile.  There is very little gains trying to upgrade the stock oil cooler.  Mazda Competition does offer an upgrade unit that is a drop-in fit.

Even with extensive mods to your FC3S, the stock oil cooler is sufficient enough to handle most oil cooling requirements.  The oil cooler should be exposed to constantly flowing air and the floorpan kept in place.  This keeps air from bypassing the oil cooler if the floorpan was eliminated.

The only other reason to remove the stock oil cooler is if you need to relocate your oil cooling system away from the front nose due to competing requirements from a larger radiator and/or front-mount intercooler.  Front bumper space becomes a premium when trying to fit these heat exchangers, especially with the stock front bumper.  An aftermarket front bumper can help solve this problem with larger ducts for your heat exchangers.

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