Startup Time

I had started using Synthetic Oils (AMSOil Series 2000 0w-30)for my 96

4-Runner, its been 10,000 miles now, i observe that now it takes a couple of more seconds to start the car than earlier when the oil was new. I also observe that the car seems to rev more than normal when cruising on the highways or going up a slight incline

Is it time to change the oil?, the engine oil ppl say it should last for 35k (there might have been a mix of normal and Syn oils when i put the syn oil in)

should i check the batteries, or starter or air-filter,

or it because it getting colder in the mornings now

anyclues are very welcome, Thanks!