I've just been checking on GM's website to see what changes were made to 2001 models to allow service inspections to be extended to 20,000 miles or one year.

The only 2001 changes due to this were; the petrol engine oil has to be ACEA:A3-98 fully synthetic and viscosity 0W/nn, 5W/nn or 10W/nn with brake wear indicators fitted to all cars.

There's obviously no requirement to use this oil in older engines like 16v Ecotec, 20XE or 8v but this does define what good quality oil is, currently.

I suggest that anyone still using 15W/40 or ACEA:A2 has a very serious re-think if they really want to protect their investment in time and/or money.