I have always used Quicksilver Premium Plus 2 stroke oil now all
I can find is the synthetic oil in the same brand. Is it safe to
use?

I use it in my 125hp Merc without any probs!!
Kenner 19' VX 125 Merc saltwater series.

I don't put much stock in premium 2 stroke oils, they are all held to a standard in order to get the tcw-3 rating. And the blends are all pretty much the same
I run penzoil premium 2-stroke in my vmax, and run it HARD, in a pinch I have run cheap offbrand and ran it HARD, no issues. And mine is a rarely serviced 450hr engine

here is a good read on 2-stroke oil, its an older aticle, but im sure the tech hasn't changed since the whole world is switching to 4-strokes
http://www.saltwatersportsman.com/techniques/understanding-two-stroke-engine-oil

2000 vmax 150 I do same as BODA, never had an issue.

The Quiksilver Synthetic Blend is the same as Premium Plus. They just went to a different label.
The Premium Plus does have different additives that Mercury Engineers decided on for best lubrication. The Pennzoil Premium is great oil, I know several people that run and love it. They say it runs a little cleaner than PP, less ash.
The cheap Wally world Tech oil meets TCW-3 regs., but I would run it. I blew a motor and know of a few others that did too running it. My thought has always been that if it's getting oil, any properly rated oil should work. I'm not risking it anymore... Powerheads are waaay more expensive than a decent oil.

This is true. I went to Academy and thought it was discontinued. After researching at the store and confirming with the smartphone internet, the label was changed. The product and skew #s are the same. This oil as always been synthetic blend, but the old label didn't indicate this. The change IMO is for marketing purposes only.