I did not know there was an oil thermostat, where is it?

So, safe to say the relationship of these two temps should be "proportional" and "close to each other".

You're right, the main purpose of blocking the main rad is to limit icy wind blowing into the engine bay, chilling the engine from all sides. It wouldn't really matter if you mounted the cardboard behind the rad to accomplish this same goal, but there's a fan there. So it goes in front.

It sounds like blocking the oil rad would be a "helper" at maintaining engine heat, but not a leader. Cardboard covering that rad is down low and doesn't do a lot for preventing cold air blowing into the engine bay, that flow is directed under the left front fender.

I think I'll continue doing what we do, and block the majority of the main rad with cardboard. I do leave an opening, near the bottom, to allow some flow through the rad down low where airflow tends to go under the block and under the body rather than straight onto the engine block. I will also block the oil rad with a similar 20% opening strategy. Of course, I have a good functioning coolant temp gauge and keep my eye on it. In the spring when it starts rising above 80 around town, the cardboard comes off.

I totally agree about the underbody tray, it helps so much with keeping road grime out, salt and water spraying up, foreign objects from taking out fan belts, oil pans, etc... and managing heat. I was shocked when I was under there one day and looked up near left front wheel and spotted a set of functioning louvers (connected to bimetal spring) that regulates engine bay temps by opening up the vent when hot. Of course this only makes sense (has a purpose) when the body tray is installed.

It would be so cool to have something like that in front of the rad. Some newer BMW's have such louvers as part of the grill, to help with aerodynamics at speed, more than for temp management.

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Cheers!
Scott McPhee

1987 300D