Synthetic oils are usually heaps more expensive, and I think are all the wrong viscosity ratings for older motors. 20w50 is pretty much standard for all a series motors unless you have a specific reason or need.

I think from meomry that almost all synthetic oils are thinner eg 10w40, or 10w15 and therefore not suitable anyways.

But as far as replacement goes, unless the manufactuer specifies a longer timeframe for a motor its kms or date.

The new fords now have sealed auto boxes that never have the oil changed in them. The box is used till it fails, then it gets chucked. perhaps engines will go this way eventually?