The period of the services is usually determined by the service indication lights, but engine oil should be changed every few years if it's not run. I would change it every three, but maybe four. I wouldn't push it much further than that with an engine like the S50/S54, as they are highly stressed when used properly.
The service indicator is actually based on mileage, and measures the amount of fuel since the last service. It's a good idea, as it means that fast driving cars are serviced more often. The average between service indications is 7,500 to 11,000 miles.
For a 25 year old car with 35k, I would hope to see at least six services based on age. These would be oil services, and at least three inspection services amongst them based on mileage. Some people wrongly assume that an Oil Service based on age will suffice for an Inspection Service based on distance. It won't.
So, you should see at least one Inspection 2. This service should have been done by someone who is competent. I would expect to see an additional invoice for valve shims (part numbers range from 11321308204 to 11321308229, 11341401415 to 11341401427 and 11341404871 to 11341404878), and the diff oil should have been changed, so there should be an invoice for diff oil.
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