5) What are (or can be) the main differences between oils of the same type, i.e. what's the difference between a 'good' and a 'bad' oil?
It all comes down to honesty really.....so beware! A good oil is what it claims to be on the can. 10W/40? Does it really pass the cold test at -25C? Quite a few I've tested do not. There is usually an API spec quoted, such as API SH or SL. These are car-based, and a good basic quality guide. If absent, leave it on the shelf, and avoid lawyer-speak:
'meets the requirements of....' or 'recommended (by whom?) for use in....'.
Then there is the 'synthetic' minefield! Provided the price hasn't been pushed up by shipping an average oil 5000miles from the West coast of the USA, you get what you pay for. The best performance oils are made
in the more developed European countries, but low price buys the cheap 'modified mineral' synthetic and not much of it, with a poor multigrade polymer. As is so often the case, quality follows cost.
6) What are the likely consequences of using poor-quality oil?
Usually, these are fairly long term, except in racing. Think of the oil as a liquid component, and poor oil as a cheap pattern spare. In a road car long-term reliability and performance retention (i.e. acceleration figures below new spec., fuel and oil consumption above) are the casualties. Particularly in a high performance or racing car,
the effects can be more immediate and catastrophic.
Thanks to John for his honest and informative advice, we look forward to the next instalment!.
Cheers
Guy.