The engine in your Combo has a code-name C14NZ (for future reference). This 1.4-litre four-cylinder unit has an engine oil capacity of 3.5 litres which includes the little extra oil required when you’re changing the oil filter as well as the engine oil.

For the record, changing the filter every time you change the oil is best practice. While some mechanics reckon a filter change is only required every second oil change, the vast majority of tradespeople recommend a filter change with every oil change. Why put clean oil through a dirty filter, is the basic argument.

If you’re planning to do an oil change yourself, make sure you buy a four or even five-litre container of oil as this will allow you to carry out the oil change as well as having some oil left over for top-ups between services.

It’s quite possible for a car’s engine to lose a little performance over time as it wears internally. When that wear occurs, the compression ratio can fall a little, the valve timing may alter a fraction and the fuel injection system might get a bit slack. All these things can lead to a slight drop off in power. Because this dwindling power output happens gradually, most owners never notice it happening.

But since the Holden Combo was a fairly low-powered car to start with, there could be some external factors that are making it feel a little tardy at times. Again, since you’re not playing with a whole lot of power in the first place (a Combo had just 66kW when it was brand-new) even tyres that have deflated over time (perfectly normal) can make the car feel a bit slower from time to time. The same goes for a brake that is dragging slightly or even having a second occupant or a heavier than usual load in the car. Even a decent headwind can make a Combo feel like it’s struggling.

The best idea in the long term is to service the vehicle properly and make sure it’s in top condition. That will rule out some of those external factors and will keep the thing in good mechanical condition so that it can muster up the maximum number of kiloWatts when you need them. You could also try a bottle of injector cleaner in the fuel tank now and then, and don’t forget the basics like a blocked air-filter, and old, stale fuel which can also rob power.

Based on your information I would suggest it would be worth around $1000.