oil for 1982 gtv2.0

Hello guys, its going to be one year since I am the proud owner of my gtv2.0 and its time for an oil change. The car had rings changed before I purchased it. It has a strong engine, with cams (don`t know the specs since the previous owner didn´t), balanced and other goodies the original owner did before he needed money and sold it to his friend who later sold it to me...
I wanted to know which oil weight would be adecuate for this engine, here in Peru we have a very stable temp ranging from 13-30 ºC. The car is only driven on weekends and sometimes on more days. An Alfa fellow told me to use 20W-50, the manual tells other oils so I would appreciate your thoughts on this one. The car currently has castrol mineral oil, it would be good for Castrol magnatec (10W-40) semi synthic oil?
Do you recommend synthetic or semi- synthetic oils? Does anybody out there use valvoline? (I was asking this because this alfa guy who has a shop used this oil)
Thanks.