Definately avoid synthetic....... it does not have the heat transfer cababilities of conventional oil that is needed on aircooled engines.
Synthetic is great for newer type "watercooled engines" but on air-cooled it's not effective and is just a costly waste.
As far as brands........ I've run Pennzoil forever and still do..... also Valvoline and Castrol in aircooled without problems...... as long as you use a "Quality brand" that your happy with and it meets the minumum use required viscosity,( i.e.... 10w-30, 30hd, 40hd, 10w-40, 20w-50).... i say go for it.... use what you like.............Just remember the oil is a major part of your cooling system.......choose wisely..... and change regularly...
Everyone has opinions about certain brands and there's been tons of research on the subject.......... here's a little help..
http://www.ratwell.com/technical/OilSelection.html