Wow, has this question been posed forever! From experience I can say it doesn't matter. Just use the proper weight for your application. Energy-conserving oils, with extra friction reducers, are not availablle above 10W-30, so if you want 10W-40, and you should, you won't have a problem. If your clutch slips it's not because of synthetic oil. The clutch is worn! I used synthetic in a Yamaha to get better shifting but it didn't matter. I got good shifting no matter what I used. By definition, all oils rated the same SAE are going to perform the same. That's why they passed the standard. I wouldn't bother putting synthetic or motorcycle-specific oil in these old bikes. I wouldn't put synthetic in a newer bike either unless you want to go a long time between changes. Otherwise, change the oil and filter every 1000-1500 miles and your bike will run beautifully. The typical bike isn't suffering from its engine oil of whatever type it may be. It's just way too old an oil and not being changed enough.
