Maybe this is a dumb question, but I'm new to diesels and have never had to top up coolant in a gas vehicle before. With all the talk on here about black sludge in the coolant, I checked mine. It's still pink but about an inch low of the cold fill line in the reservoir. Should I just top it off, or be concerned with a leak being that low? I have 10,000kms on it and haven't done anything other than an oil change so far.