Goughy, your D4 7U cooling system holds 42 litres of coolant, if the system has been fully drained.
Firstly, how rusty was the coolant originally? If it was really rusty, a fill with water with a cooling system cleaner added is the go, before you add any new coolant with inhibitor.
Run for the recommended period with the cooling system cleaner in it (a few hours at engine operating temperature), then completely drain the system again (remembering there's a block drain as well as a radiator drain) - flush with clean water - drain the clean water - then buy some good quality, "brand name" coolant, that contains glycol with corrosion inhibitors.
The coolant comes in "ready-to-use" formulations, or in bottles that need to be mixed with clean water, usually 50/50 mix.
ALWAYS use rainwater or distilled water - don't use straight tap water, because no matter how good your tap water is, it contains minerals and salts to varying degrees, that facilitate corrosion in cooling systems.
Your local auto parts store will have a range of coolants available in 5L and 20L containers. My recommendation is to buy the long-life coolant "concentrate", and mix with distilled water.
Modern engines use Organic Acid coolants ("OAT"). Older engines used Inorganic Acid coolant ("IAT"). The OAT coolants are better for the modern engines where a lot of aluminium/magnesium alloys are used in conjunction with cast iron.
But the IAT coolants are better for the older engines where very little aluminium/magnesium alloy was used in the cooling systems.
As the D4 7U is a largely cast iron cooling system, with no alloys, an IAT coolant is your best choice.
You won't need any coolant with anti-freeze added to it for your location - unless you're in a particularly low spot that sees regular temperatures below freezing in Winter.
Here's some useful coolant advice from GMB, the Japanese aftermarket manufacturer of water pumps.
What's The Difference Between Inorganic And Organic Coolants? Find Out Here.
gmb.net
