There should be a restricted oriface right at the turbo, look at your turbo where the oil goes, and you will see only a tiny hole for the oil to go in. No need to restrict it anywhere else.

I think the metal on the back plate of a non turbo engine may be a bit thin to allow drilling and tapping for an oil line. You may need to weld a bung on it.

I don't see any need to cool the return from the turbo. Although you may want to change the oil cooler for one from a 300. So far I have not. Perhaps put a oil temp gauge on, this would be in the oil pan.

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1977 240D turbo