Oil temp and coolant temp run about the same in the car.
So if your coolant system is excellent and you're not running hot chances are your oil is cool enough. Ambient temp and engine load would have big weight in making this decision. If you live in Arizona and frequently tow up hills... Well.. Put the cooler in.

I live in Canada and block my rad with cardboard in the winter... I also block my oil cooler with a piece of cardboard. I am pretty sure I can get away with no engine oil cooler. Also don't really need AC up here except for maybe 3 weeks of the year.

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Cheers!
Scott McPhee

1987 300D