That's the "hornets' nest" question and you'll get a wide variety of answers.
I appreciate there will be some climatic differences but out here in Australia my mates and I are using the closest Castrol gearbox multigrade we could get to the recommended grade. I think it's about 80w at the higher end but will check.
As for petroil mixes (this is where the fun starts) I run in on 25:1 mineral oil for 500 kilometres and then change to 50:1 Motul synthetic although any good synthetic is fine. Our bikes have collectively done over 10, 000 km at the 50:1 ratio and we recently stripped one down (3K on the clock) and there was no appreciable wear. One tip. Synthetics are generally a pig to mix, dissolve a small amount in a petrol drum then add the rest of the fuel. Agitate well.
The bikes run noticably better on the 50:1 ratio.
One last comment, whether it's a result of modern fuels or changing ratios, all our bikes have had the carb needles changed to the top notch (effectively dropping the needle down one notch). This gets rid of the bogging down when the throttle is opened.