It sounds like you have a breach of combustion pressure into the cooling system. Unfortunately with these cars, that usually means cylinder head cracks rather than 'just' a head gasket failure. Leaks from cooling system hoses and fittings indicate of high residual pressure in the cooling system. The more pressure you lose, the less stiff the hoses will be. You might buy time by leaving the radiator cap loose such as on the first catch. This will prevent build-up of pressure but severely reduces the ability of the cooling system to dissipate engine heat. Hopefully that's not a problem this time of year in the US. Check the coolant level every time before you start the engine if you can and keep one eye glued to the temp gauge.

IIRC Mobil 1 15W-50 is not a Diesel rated oil. Get Mobil 1 turbodiesel/truck 5W-40 (5W-40 Diesel Engine Oil | Mobil 1™ Turbo Diesel Truck) if you must stick with Mobil 1 but get a Diesel rated oil.

Sixto
87 300D