Q: I have a 1993 VW Golf GSX 1.8. I always used leaded, 93 octane petrol until that was phased out. I have been advised that 93 unleaded is OK (although my mechanic said I should use 95 octane). I phoned VW careline and was told this model is compatible with unleaded 93 octane, "as VW had anticipated the change". Some other makes would have to use an additive. - Lynton

A: Lynton, in a nutshell, if a car has an aluminium alloy cylinder head, (such as your '93 Golf certainly has), you can use unleaded petrol without any risk to the engine.

If, on the other hand, the engine has a cast iron head, such as lots of older (pre-1980) cars and some later bakkies had, you will probably have to stick to lead replacement petrol while that's still available.

The reason for the different fuel requirements is that, on cast iron heads the valve seats were usually cut directly into the metal of the head - cast iron was hard enough to allow this.

But aluminium alloys are too soft for that.

So manufacturers were forced to install special, hardened valve seat inserts when they switched to alloy heads.

The widespread adoption of "leaded" petrol had a fortuitous side-effect: the lead compounds formed a coating on the valve seats which provided sufficient protection to prevent the seats on cast iron heads from being worn away.

Even with leaded petrol, alloy heads were too soft, hence the need for hardened valve seats.

When leaded petrol was discontinued, the hardened seats on the alloy heads could cope without the lead residues.

But the seats on cast iron heads were now in danger of being worn away. There is some debate about how real this danger is.

From what I've heard, it's only a serious worry when the vehicle does a lot of fast, long distance trips.

On some cast-iron heads, hardened valve seats can be retro-fitted but on others this is not possible. Owners of such vehicles are watching fuel developments with misgivings, knowing they are at the fickle mercy of profit-hungry oil companies.

It's a matter of waiting and watching the valve clearances closely, because valve seat recession will first show up as abnormal changes in valve clearances.

Q: I am amazed at the general trend among dealer workshops to simply ignore all recommendations for periodically flushing out the old brake fluid in the hydraulic system and replacing it with fresh fluid. Is it no longer necessary ? - DIY Veteran

A: The need for periodic renewal of brake fluid has not fallen away. Ordinary (alcohol-based) brake fluid is still hygroscopic, meaning it absorbs water vapour from the air.

The latest DOT 5.1 formulation may do so more slowly than the older DOT 3 and DOT 4 fluid, and it may disperse the absorbed water better than the previous formulations, but the process still takes place.

And water in brake fluid, even in small quantities, has all sorts of undesirable side effects - such as corrosion in wheel cylinders and on caliper pistons; and it lowers the boiling point of the brake fluid.

This issue deserves fuller discussion, and I shall return to it in a future edition of MotorMania.