Thinking of visiting Portugal at early-mid October, is it a good season to be in Porto and Lisbon that time? Just googled the average weather, it said precipitation is around 160mm in Porto and 90mm in Lisbon that time, is the more like drizzle from early to night, or it's a shower and after shower it may turn sunny? Anyways, is it a good season to go?

Googling average weather will give you the same thing as asking on here. Nobody can tell. Those are averages.
We spent 2 weeks in Lisbon and the south in October a few years ago and had warmth and sun. It could just as easily been rain. Offhand, I would say the further north (Porto), the higher chances of rain, but it's a roll of the dice.

Can be really nice, and can be really crappy. Especially Porto, Lisbon should be almost guaranteed pretty good.

It depends but in Lisbon is allways better weather than Oporto. The southern you go, better weather you get..some years ago i went on beach on Algarve at October...
In Lisbon it can be sunny all day long, but with windy nights.
In Lisbon, rain in October, usually is a morning or night shower.
But again its an average, it depends, but came and enjoy..
Well, apart from all the above people to be right about the weather, if it rains it won't be nothing relevant, somo showers ... nothing that your rain gear will not handle.
Being in Portugal you might want to take a look at my blog where you may find nice things to do: http://walkinginportugal.blogspot.pt/
Do not hesitate to call in case you need some help.
David