Do you know the location of your Oil Cooler?

As an example my 84 300D has a Coolant Radiator that has the Transmission Cooler built into it but has a separate Oil Cooler Radiator.

A lot of vehicles have the Transmission Cooler and the Oil Cooler built into the Coolant Radiator.

So if your Oil Cooler Hoses go into the Coolant Radiator that a likely source of the Oil getting into the Coolant as the Oil pressure is higher then the Coolant Pressure.

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84 300D, 82 Volvo 244Gl Diesel