1996 Trident Misfire

Try checking over the carbs and balance and if this doesn't sort it then check to see if you have something you have disturbed or going faulty in the ignition system, as for oil going one grade thicker will not be the cause of the oil light problem if anything it will raise the pressure slightly, the oil light being on basically tells you that the oil pressure is below the minimum pressure threshold so when hot it maybe just there is residual pressure in the system ...or the sensor is faulty.

 

My '95 Trident 900 had a slight vibration around 3,000rpm when it was relatively new. Now, with over 54,500 miles on the clock, it is creamy smooth. As has already been said, balancing the carbs could cure your rough patch. If the bike has been standing for a while, stick some Redex or other similar fuel system cleaner in the petrol. This really does make a difference with these bikes.
I've used 5w/40, 10w/40 and 10w/50 semi and fully synthetic oils in my engine over the years. It uses very little between changes but seems to be liking Motul 5100 10w/40 semi synth at the moment - first time I've tried it and will be using it again at the next oil change.
Just another thought - when were the valve shims last checked? Mine improved noticeably when they were done last time.

 

Pilot jet blocked. Start from with warm engine but cooled down pipes. Start and hold at 3k for a few seconds diff temp pipe will tell you which pilot is blocked.
You will need to poke some copper wire through to clear. Too small for ultrasonic and compressed air will fire it out your hand never to be seen again. Don't ask me how I know.