Mineral 15w40 or Semi Synthetic 10w40.

Semi syn will give u better mileage compared to mineral bcz mineral is thicker, thus having heavier FC. But my advice for u, don't ever go below 10w40 for old engines bcz they need thicker oil to preserve the engine. Just my advice though. U can still go with semi syn for better FC

confession,

if u just bought the car. its good to sent it to your mech to check and do a routine maintenance...so you will know ur car condition.

proton normally 4Litres +/- only

old car dun need go proton...go to a mech that you familiar, or ask your fren to recommend(if any)...

if i am u..i will

flush n replace new engine oil
flush n replace new gear/transmission fluid
flush n replace power steering fluid
replace timing belt(full set with water pump) - for safety reason better do this
fan belts - to check
flush n replace new brake fluid
if brake disc rotor n pads still ok, then can use back
if manual, flush n replace clutch fluid
flush n replace radiator with better coolant
clean n service battery terminal for better contact