On my 84 300D The Oil Filter Housing has a thermostat in it. If someone would post a site that has the parts breakdown drawing you could see if your year and model has a thermostat in the oil filter housing.

Having a thermostat also implies that the Oil flow is restricted till the thermostat starts to open mean the Oil has to be hot enough to open it.

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84 300D, 82 Volvo 244Gl Diesel