2x per year is resonable.

In tradeschool I was told that some of the additives in the Oil get depleted after 6 months even if the vehicle sat in one spot for all of that time.
However, that has not been proven in any experience that I have had; and that was what Oil was like back in about 1975.

Sending your Oil to a lab is like twice the cost of a typical Oil Change. But, you might try extending the Oil Change for one year and then sending in an Oil Sample to the Lab.

If the the lab tells you the Oil Sample is still good to go you can increase your Oil changes time to once a year and in the long run save youself some money and the labor on the oil change.

Someone who knows about synthetic Oil might want to comment on if full synthetic Oil would allow the Oil change to be extended beyond a year.

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84 300D, 82 Volvo 244Gl Diesel

With these cars and I think IDI diesels in general, it isn't the oil wearing out that requires the change, it's soot loading reaching max thresholds. So whether dino or synthetic, without significant extra filtration to remove the soot, OCI will remain the same.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap

Agree it is more about the soot loading so I'd stick with dino and just change it. Plus with 274K on the clock the rings are probably letting a little extra soot into the crankcase. The oil filter on that car is fun to wrestle with, a primary, secondary, can to clean and finicky bolt to get started.

Also agree the lab test costs more than oil change...go figure!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

One would think that more use equals more soot loading. If that is the case driving only on the weekends would equal less soot loading per time period.

An Oil Analysis would show the soot level and tell you if that is a problem.

It is a fact that private parties and money making trucks are saving money by extending Oil Changes while using synthetic Oil. You initially need to use Oil Analysis to determine how much use you can get from your Oil.
Of course in industry the Oil Analysis is also used to find evidence of pending mechanical or coolant leak issues.

I agree with the extra filtration and that I why I have a bypss Oil filter on my Mercedes.

In short an Oil Analayais could tell him how much use he can get out of the Oil and then having some actual facts the owner could make a decision.

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84 300D, 82 Volvo 244Gl Diesel