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Yes, the rest of the motor can last up to 150,000 - 200,000 miles other than the intermediate shaft bearings if cared for properly, but the exhaust valve guide tolerances will be worn out by 25,000 mile.
I know from experience and if the car has been driven like ALL 930's have been driven.. I totally gaurentee it no matter what fancy modern alloy guides you stick in there.
The exhaust guide oil is upside down and it has a tight oil seal on it so it doesn't get enough oil until the engine is shut off and oil from the tank slowly drains back into the case and the lower 1/4-1/3 of the lower half of the cam towers.
The exhaust guide and valve is downstream in the already heated airflow and very little air cooling is happening there.
Add to that the very high exhaust temps from the exhaust backpressure caused by the turbo (often around 25psi in the exhaust header primaries at 14psi or .9bar boost) and vacuum retard for emissions control and catalytic converter operation and the exhaust guides are worn out on average between 20,000- 30,000 miles.
Doesn't mean they won't keep working but if you wobble the valve stem sideways while doing a valve adjustment it will move sideways alot even after only 25,000 miles and by spec the valve guide is worn out but because it's so expensive to replace them so people ignore it and keep driving until other things make them decide to do a top end or complete rebuild.
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