An oil cooler is not needed unless high oil temps are an issue, and would be detrimental to engine life if the oil is not allowed to come up to full temperature (210-250F) on a regular basis.
The large remote oil reservoir is already too much of an oil cooler for shorter trips. Unless you live in a desert climate and take very long drives, you should not need an oil cooler. But if you must fit one, do so with one that has a thermostat to bypass the cooler until the oil comes up to temp.
"Every biker" is not a good source of petroleum engineering information. Seek out information from an actual petroleum engineer or read things written by them.
