My Prius C has a best-tank mpg of 61. It was a lot of work, but owners with a better commute profile can easily get better. Your mpg would largely depend on your drive profile. If you do more highway you'll get around 48mpg. If you drive in the city you'll get more like 53+. Careful and artistic driving will net you much better. Imprudent driving, cold weather, and lots of heat/ac will drop the mpg's precipitously.
I've always been a big fan of 5k mile oil changes and will continue that for all the other cars. I use PP or M1 for all of them. I don't use conventional any more. It's not my mission to eke out the last tiny bit of performance/value. I don' care that much about it, and 5k on syn is fine by me.
However, on the C I will scoot that out to 10k. I've seen some good UOAs and it seems legit. However, 7.5k is interesting, too. Over 10k miles, I'll probably have 25%-ish of that in electric mode. that would be about 7.5k. If I went 7.5k it would be about 5k miles of engine use, which would be perfect for old prejudices to be made happy.
Decisions, decisions. I try real hard not to agonize over these things but sometimes I just can't help it.