Oil change advice for interval and type

[ilnot1]

I have a 2010 3-Door Liftback and I just reached 105,000 miles. Up to this point I've been doing conventional oil changes at the manual prescribed 5,000 miles. So I live in the Northeast (Philadelphia) and probably do 60/40 city/hwy. I don't think I am a serve driver, maybe the high end of normal on the highway. I am making a job change so I'm only going to be doing 3-5 thousand miles a year going forward.

Questions:

1) When, if ever, should I start thinking about high-mileage oil?

2) What is the best viscosity level for my area and driving type?

3) I understand good synthetic is slightly better in terms of deposits and fuel efficiency than dino. If got a 10-15 thousand mile synth it seems like a bad idea to only change my oil once every 3-4 years. What is the maximum time interval between oil changes for both synth and conventional no matter the millage driven?

Thanks