[Fr. Frank]

The OMC oil is good. I have two Mercury motors, and I run the Premium Quicksilver synthetic blend oil. I don't recommend off-brand oils, but Pennzoil is good, considering that they actually make the OMC 2-stroke oil to BRP's specs. Quicksilver oils used to be made by Texaco, but I believe Chevron now makes them to Mercury's specs. BTW, TCW-3 is a minimum standard for oil, not a guideline. But not all motors can use TCW-4.

As to the gas-oil ratio, if you run above 4800 rpms more than you run below, you should mix 40-1, and if you constantly run in the 5600-6100 rpm range (or higher) go one heat range colder with your plugs to avoid detonation. (The price more that doubles for the different plugs, and you'll probably have to special order from a marine racing supply company. For instance, the Denso SF51 plug retails locally for $8.42, as opposed to the SF50 for $3.77)
If you continue to run just the stock carbs and mostly below 5600, just run the BUHW plugs from NGK, or the SF50 from Nippon-Denso. I prefer these to Champion for quality control reasons. If you have to go with a colder plug, go with the Denso brand.
If you run mostly below 4800, mix at 50-1. If you have the original carb jets, never mix heavier than 40-1, or you'll end up leaning out your fuel-air mixture, and risk blowing the motor. Racing applications mix at 36-1 or 24-1, but they run larger jets to avoid lean conditions. The downside to that is horrible economy.

Hope all that helps.