I am confused by the answers I am getting from different garages and forums about oil.
We have a 1992 Peugeot 205 1.9 GTI which was made to use standard oil.
We converted it at the main dealer to semi synthetic. The car does 5/6k miles per year and we understood that unlike standard mineral oil it does not need to be changed every 6 months but at 5/6k intervals, ie annually.
This seems to be working OK after a further 12 years usage.

We also have a Honda NSX. This was on fully synthetic oil when I bought it 3 years ago.
However I have now been told by the main dealer that it was originally made for semi synthetic oil, but they put in whatever oil the owner specifies.

I have always believed that fully synthetic oil was more viscous and because it had more cleaning agents it was not a good idea unless the engine was made for it originally.

I have seen tests in a classic car magazine where three conrods were coated in the three types of oil. The semi synthetic oil stayed on the conrod longer than the mineral oil, but the fully synthetic stayed on a very long time. But I am not sure what that proves.

Is it a rule that semi can replace mineral and fully synthetic can replace semi?

Should I return either car back to the original oil?

Are there true down sides with using “better” oil?

I also have a CLK 320CDI which I follow your advice and change the Fully Synthetic oil at much more regular intervals.