It's common for people to put thicker oil in thinking that it's helping with oil temp. Recently had someone ask the same question as you and a tuner told him to use 10w60. I said stock engine should be like 10w40 just use good quality oil.
Ended up I told him to ask opie oils as they are the experts and he didn't know who to believe, afew days later he returned a message and said that I was correct.
Spark plugs any grade 8 plug should do really, I use iridium in mine.
I use honda mtf in my box and it's fine.