Motorcycle transmissions can be fickle , so it might take trying a few until you find one that your bike really likes. As Okie suggested, Dexron ATF is an excellent starting point, given that automatic transmissions in cars share a lot of the same characteristics as the wet clutch transmissions we have in bikes. A high quality 10W40 automotive engine oil will also work well in a lot of cases. You can buy motorcycle specific "transmission gear oil" from any number of manufacturers that will do a great job but tend to be much more expensive, Don't be confused by the gear oils odd 75w or 90w type ratings, they have the same basic viscosity profile as a 10W40 engine oil.
For newer bikes synthetics tend to be the best choice whether using ATF or engine oil, but they don't always play nice in older bikes with older seals and clutch materials. Unless you are sure about synthetics it's probably best to stick with mineral based oils for an older bike like yours. There are lots of great options. I've run Shell Rotella 10w30 in lots of transmissions over the years with great success.
Those old Can Ams are really cool bikes glad to see you are keeping it alive. Make sure and post some pictures of it.