Just about all engines on this planet are ultimately ....... AIR COOLED ..... bar outboards and other engines that pump water from a large source ...... even then you could argue that THEY (river/lake/sea) are air cooled ... LOL ... is the liquid (oil/water) not cooled by heat dissipation into the air ?????? Just cover the fins on your cylinder or radiator and remove the fans (mechanical and thermo) on "water cooled" engines and see how long they last while being revved in a low gear , pulling weight or even at a stand still . IF oil coolers do nothing , then why are all the manufacturers fitting them to bikes and cars ...... and even to automatic transmissions and differentials .
The first 4 posters in this thread (including Hillz) just about covered everything ........
It's a fact that a vehicle not fitted with fans must be kept moving at a decent speed to get adequate airflow over the cylinder or thru the radiator for any REAL cooling to occur . That's why mowers and stationary engines have magneto fans and shrouds to direct air flow at the head and cylinder .........
You CAN fit a cam end plate oil cooler adaptor to a Ducar BUT you have to be really careful to check that the oil is flowing out of the outlet hose and that there isn't any air trapped in the cooler or lines .... the hose connector fittings can be screwed in too deep and cut off the flow .... ALSO , coolers and radiators rely on fluid pressure to work properly . Heat relies on PRESSURE for efficient transfer ....... so the inlet nozzle to the head from the output line of the cooler needs to be sized to properly maintain pressure in the cooler .
Case take off coolers do that for you because there is an oil flow restricting nozzle in the oil gallery AFTER the take off ports and just before where the gallery enters the cylinder ..... the oil flow is valved to maintain high oil pressure to the crank shaft ......... so there's also high pressure oil going thru the cooler which will make it work much more efficiently .
Whenever you fit an oil cooler , you should make sure the system has been bled and that oil is making it to the cam lobes BEFORE even thinking of starting the engine . The same goes for when doing oil changes ...... DO NOT kick the bike over while there is no oil in it (even with the ignition leads off or the plug out) because you will lose the oil pump priming . If you do turn the motor over ... you will then have to kick it over with the plug out until you fully bleed the galleries and recheck that there's oil making it to the cam lobes . XR75's have a bleed bolt on the side of the head that you remove to verify top end oiling ...