Interesting questions. I think that if it is PVT and the MR does not have inscribed a 50hr oil change then it is not legally required.
For that matter it only requires as a bare minima an annual inspection. The engine manuals do state oil change intervals but they are not a mandatory thing depending on the system of maintenance applicable.
I think......but I am not 100% sure and I bet you get lots of opinions.
All the legal BS aside, lets examine some reality of engineering.
Oil changes are not as necessary when the oil is not old, is regularly topped up(progressive replacement) and the plane is flow a lot. That said, depending on how the engine is operated (read mixture management) the oil deposits could vary significantly. This is why oil change intervals are recommended depending on usage. In particular H2O which helps form acids.
Next is spark plugs and magneto's. If they are fine wires, then no big deal. 100 hrs is fine. All they need is a resistance check and checked for cleanliness. If they are Champion, this is far more critical.
Air cleaners, this is a 100hrly thing but depending on locations this may be prudent at 50 as well.
Is there an engine monitor? Is it downloaded regularly and the data examined. Is it examined by someone who really knows what they are looking at? (only 1% of pilots and LAME's are in that boat)
Are the cylinders borescoped each 100 hours. Oil analysis as much as this is a variable test?
There are so many variables.....if you wanted to talk in private I would happily help point you in the right direction but here could be a wild ride.
Last of all, remember this. Just because it is legal does not mean it is prudent. All maintenance should be viewed with this integrity test. Does it do everything it should and nothing it shouldn't?
Feel free to PM.
