Situation Overview
Oil coolers work perfectly well when a vehicle is moving at speed but in slow and standing situations such as traffic jams then the oil cooler does nothing at all as it has no cool air passing through it. The main problem is oil can reach a far higher temperature than water, and with no air flow through an oil cooler when sat in traffic it will take so much longer in between traffic queues to bring the overall engine temperature down.
Remedy
The solution to this is to install a fan on the radiator either on a thermostatic switch or a manual switch on the dash which will draw air through the oil cooler when you are not moving thus keeping oil cool in just the same why your coolant radiator fan works.
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