theres like a long list of "what oil" threads in the search. here's one of them, covers some other fluids as well.
general consensus/common preferred brands:
castrol gtx = good conventional motor
royal purp = good synthetic motor
redline = good syn motor/diff/tranny
neo = good diff/tranny
amsoil = severe gear(diff)
motor - depends on temp. 10w-40 , 20w-50 are common. but everyone uses what they prefer normally
tranny/diff - 75w-90 / 80w-90
here's an excerpt that answers one of the OP's questions:
Originally Posted by Icemark
Depends on the temperatures in your area.
In starting and operating temperatures below 20F, use 5W30
In starting and operating temperatures between 0F and 80F, use 10W30.
In starting and operating temperatures above 30F, use 20W50
As far as brand any good quality oil that meets the API SL or SM standards (or better) should be fine.
In my area, I see winter temps in the low 30's when starting in the morning, so I use 10W30 in my FC in the winter, but in summer I only use 20W50. because I never see temps below the 50's when starting.
Using the correct thickness will ensure long bearing life. Too thin and you wear though the main bearings as well as loose shear strength.