I have the Connector that goes to the Oil Filter Bottom and it still has a piece of pastic line on it.
The ID is 3mm and the OD is a hair under 5mm.

Ther is a lot of Platic Gauge lines available and they will hold the pressure. However, that does not mean that the ID will be correct or that if the ID is correct when you shove it over the Barb on the Stock Connectors it is going to hold.

There is also one of those skinny plastic lines that feeds the Fuel Injection Pump Oil just waiting to crack and spew Oil.

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84 300D, 82 Volvo 244Gl Diesel