Help me understand why you say it is tired. The additive levels look fine and the silicon levels are not out of line given the significantly higher mileage than previous changes. Base number seems good for a while yet. Seems to me a different oil is unlikely to affect silicon. Presume you have looked at the air filter and the mice haven't eaten off the pleats like they did on mine in its early life.
Also I don't understand what 'mexican emissions' means and why an older formulation oil is suitable. I guess you are saying you have nothing to contaminate with heavy metals and ash so why not have an oil with more of them?