GS150 has a small glass window on the right side of engine, you can check the oil level and colour through it. To check the quantity you have to read the manual and derive your own level mark after checking from that.
Whatever oil you use, buy it from a reputed fuel station (or main dealer/distributor, if it is not available at fuel stations). I think the problems about Zic maybe because of bad dealers, I have never used Zic myself. But in Abbottabad, almost every Suzuki Bolan and Ravi user uses Zic in the winter. Nobody uses petrol and the thinnest Zic makes cold start easier at around freezing temperature, with CNG. I haven't heard many complaints from them and almost all were about not buying from their regular dealer.
Generally, fully synthetic oils are thinner at normal temperature, semi-synthetic are medium and mineral oils are thick, they all perform within the specified range (10w40 or 20w50, etc.), some prove better for one and the other proves better for the other person/bike/car.
I chose Havoline because it is available all across the country at Caltex stations and my previous experience with Havoline Formula3 was excellent. Shell and Castrol also have the same advantage. If you don't travel extensively then it doesn't really matter.