If you have buzzer or know oil pressure problems when hot try 15W-50 Diesel oil in summer (rated for turbo diesel)
If you have buzzer or oil light and known oil pressure problems in winter then try GSF's Morris 20W-50 Diesel, blue container (not turbo rated, but then compared to an engine rebuild, who cares, it seems to work, just change it more frequently)...
I can't confirm whether your water needs topping up, only you can do that, as I don't know where you live, and if I did, wouldn't want to be caught with the engine flap open

but when you need to, use:-
Anti-freeze/water up to 50/50 ratio
although if its just a little can use plain water