Not sure what you mean by grade. Either conventional or synthetic oil will work. Some folks swear by motorcycle specific oil, but having looked at test data, I'm using regular automotive oil in my bikes. The most important thing is changing it regularly. Brand isn't all that critical to me.
People in this forum recommended Rotella oil (available at wal mart), I got the synthetic. It's fairly inexpensive compared to other synthetics and I definitely noticed improvement, especially in my previously neglected VN700.
Only thing you really gotta look out for is making sure it is NOT "Energy conserving" oil. Look at the bottle closely to make sure it doesn't say it's energy conserving. Those additives don't go well with our clutch system.
I've been using Mobil 10w40 4T motorcycle synthetic oil here in the desert in NM. I use it all year, even at 100+ degrees. I used to use Amsoil but I found the Mobil 1 was easier to get. I am very satisfied with it. I also use Lucas synthetic in the final drive.
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