Sorry for this question there was a study a good number of years ago by an oil company to look at how long engine oil would last if the person was only doing a small mileage every year. I think it was called Aunty Betty or something similar but a google search is not being productive.
I have never seen the study but it was mentioned on here a few days ago. They looked at the engine condition over say six years and advised some sort of extended change. If I remember it was written by say Exxon or one of the other majors and published by one of the societies like the SAE or equivalent.