I have recently had an experience which suggests that either the correct viscosity grade for engine oil is grossly overrated, or someone's nose is longer than Pinocchio's.

I went to the agents for my car (a 2009 model) to buy 5 litres of engine oil. The guy at the spares counter supplied Shell Helix Ultra Extra 5W 30.

About to change the oil, I noticed a factory sticker on top of the radiator housing which said that only 20W 50 motor oil should be used.

When I checked the owner's handbook, it said that the manufacturer of my car recommends Castrol SLX Professional 5W 40 for all its petrol engines - 5W 30 is recommended for diesel engines only.

When I spoke to the spares manager, he told me that head office recently told them to use 5W 30 oil on all vehicles of my particular make.

To be absolutely certain, I also spoke to the workshop foreman. He told me that they now receive 5W 40 oil in drums, for use on all vehicles.

As can be seen, I had three different opinions at that stage: spares, workshop and factory. I then sent an email to the manufacturer's technical department. Their answer was to repeat word for word what the spares manager said. This didn't really help to clear up my confusion. - Oil Be Damned

Well, OBD, I was going to tell you to stick to the tried-and-tested 20W 50 oil specified on the engine sticker, until I mentioned your letter to a knowledgeable private mechanic.

He maintains the reason why the manufacturer in question has gone to the Shell Helix Ultra Extra 5W 30 oil for all their petrol engines is to counter the widespread problem of sludge formation.

Whereas sludge used to be an occasional occurrence which could be prevented or cleaned out by means of an engine flush at every oil change, the problem has mushroomed.

It's now found in many engines. And it's bad sludge - thick, hard sludge that cannot be shifted by an engine flush. The only way is to drop the sump and wire-brush it off.

It's not something that can just be ignored, because it will build up until it clogs the oil pick-up in the sump, whereupon oil starvation will follow.

The reason for the sludging problem suddenly escalating is not clear. My mechanic pal speculated that it's something in the petrol formulation produced lately which reacts with the oil in the crankcase when unburnt petrol migrates past the rings.

But whatever the cause, the problem must be a worrisome one for the motor manufacturers. It could well be the reason for the switch to fully synthetic 5W 30 oil.

This oil will probably be significantly more pricey than good old 20W 50 mineral oil, but I suggest you follow the manufacturer's recommendation unless you run into problems with excessive oil consumption or oil leaks. Console yourself with the knowledge that it could have been worse: a certain German luxury car maker specifies Castrol Edge Professional TWS 10W 60 oil for their sporty models. This oil is available from their spares counters at R152.86 per litre.

As a general rule, owners of cars using recently-designed engines should stick to the oil recommended by the manufacturer in the owner's handbook.

Turbocharged engines in particular, have very specific oil requirements and owners are courting disaster if they deviate from the factory recommendations.

I don't know where the 5W 40 recommendation in the handbook came from, but perhaps the manufacturer felt it was close enough to 5W 30 to give permission for its use.