It's response's like this guy Tim that are making me not even want to try and answer some of these newbies questions. You try and give a experienced, educated response to a deserving question and Boom "any oil is good enough for these bikes" and did ya see this movie yada yada!!!!!!!!!!!!!!!!!!!!!
I'll go along with BJs basic premise, but try to tone-down the frustration expressed over the bad advice given in that earlier post. To mix metaphors in a blender - you can lead a horse to knowledge but you can't make him believe it (or something:4
Everyone starts off as a newbie. Yeah, the same questions keep getting asked year after year. IMO, when it's the newcomers asking them, it's a good thing. When a wack opinion is expressed, it's nothing to do with the new guy who posted the question, all the more reason to set things straight.
Depending on the model in question, the clutch cover will say either .7L or .8L and specify SAE 30W above 60F or SAE 20W below 60F. The CT70 motor is, for practical discussion, little more than a Z50 with a different upper end. So the same oil requirements apply to both.
You should use a bike-specific oil formulation, intended for use with a wet clutch. Some synthetics are listed as such. I'd avoid them. There have been a lot of problems with clutch slippage, directly attributable to synth oils. Second, automotive oils no longer contain ZnDDP, a high-pressure lubricant that protects the cam/rockers, gears & shift forks. As long as it's bike oil, of the proper viscosity, keeping it clean matters more than the brand and synth vs conventional, so spend the money on more frequent oil changes.
Clean the oil spinner. No one ever did this when these bikes were new and that's why most of them are smoking and way down on compression by 2000 miles. First cleaning should be done at 300 miles; I recommend 1000 mile intervals, or yearly (whichever comes first) after that.
One area where modern tech does play nicely with old is in oil tech. I don't think these engines would have lived long on the 10w40 available 40 years ago. But that's one of the biggest improvements. Oils have far better thermal stability. For that matter, straight 20W or 30W bike oil isn't all that common in 2010. BJ has demonstrated, pretty convincingly, that 10W40 works. In my own bikes, I run nothing but 20W50 Honda GN4 oil. (I only ride in t-shirt weather and give the engine a long warmup) It's probably nothing special, I'm just old school and prefer to stay with what's been working for a lot of years & miles.