There's no reason why you can't use synthetic. Except. Synthetic will loosen old deposits, which might cause problems. But you've already used it, so that's not a problem. And synthetic is better at finding leaky gaskets.


Nobody can just guess the best OCI for your car. What I would suggest is driving about 5000 on the synthetic, and sending a sample to Blackstone. They'll do a chemical analysis, and let you know whether you can push it further. Just include a note with your sample explaining what you are trying to do, and they'll return a personalized analysis:


https://www.blackstone-labs.com/


You have to order the "free" test kit before you change oil. The kit is free, the test isn't.