I can take a whack at question 2,3,4
2 You will notice no measurable increase in power or mpg. If it did it would be extremely close to 0%.
3 A regular oil change with filter will suffice.
4 No, it won't make any difference necessarly going to synthetic.
I am a strong supporter of sythetic oils with the primary benefits being cost.
Longer drain intervals and longer engine life and protection being the causes of the low cost.