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After some major upgrades a couple of years back and getting a bit old for heavy, powerful, modern bikes, I've started using my CB77 as my main road bike. I've done some good performance mods so keeping up is no problem but I'm worried about wear on start up and initial running on what is a 60 year old design/bike. I'm running on Duckhams 20:50 which has good Zinc content and I may change the oil at 1000 miles. Anybody have any feelings on wear of this engine when used regularly with no winter riding. It's a real pleasure so far for sure.
The alternative is using modern fully synthetic oil which keeps the critical loaded areas wetted between rides.
I think people fret too much about motor oil. Honda's recommendation (the values cast into the engine case are far from the complete story) in their later owner's manual is for a multigrade with 5 to 20 as the low number and 30 or 40 for the high one. So 5w30, 10w30 or 10w40 would be suitable. Back in the 70s I used Castrol 20w50 but that is too viscous, especially on cold mornings. I even used it in the winter on very cold days (-10 to -20C). Fortunately the engine survived the abuse.
My suggestion is to use a high quality motorcycle oil to prevent clutch slipping which can happen with newer automotive oils. Given the short residence time of oil in the engine (1000 mi) synthetic oil won't be of much benefit but it won't hurt either.
All this being said, by far the most important thing is to change the oil as you are doing and keep the level up. These engines will last a long time if you do these things.
Re: oil. The original specs was for 30w in the summer (20w in the winter). I've read that the needle bearings used on our cranks don't like too thick of oil. The oil will just be "pushed" along and won't lubricate. A good 30w detergent (if you can get it) is still a great oil. But since the 30w oils are usually non-detergent lawn mower oils, you probably will need to use a modern multi weight oil. I've been using 15w40 Delo 400 diesel oil for many years - I live in the tropics, always above 70 degrees. I just pulled my heads off because after 60 years my head gasket finally went. Cylinders are in very good shape, still hone markings in them, pistons are fine, and rod bearings seem to still be good. I've never had clutch problems - still the original clutch. The bike has 40,000 miles on it. You could run a motorcycle oil for 3 times the money, or a good quality diesel oil for a lot less, which is probably just as good! That's my two cents!
You are assuming that the top engine cover represents everything that Honda had to say about this but as I said, the owner's manual gives other recommendations. The hardware store SAE 30 oils that I'm aware of are intended for the lawn mower market and to put one into your engine would be to abuse it. They have SA or SB ratings that were current back in the 60s. You can do much better now. The SAE 15W40 that you describe seems to be an excellent choice which is confirmed by your experience.
Just to be a bit pedagogical, there is no "30w" grade. The W stands for winter and they are tested differently than the summer grades such as SAE 30. The grade SAE 15W40 indicates that the oil meets the requirements of both SAE 15W and SAE 40, allowing year round use.
I believe that SAE30 oil is for hinges and as soon as better oil came out in the form of multigrades in the 60's they were Honda recommended for use in the CB77. I believe personally that the oil has to be a 10:40 or 20:50 multigrade especially if used at all in winter. I also believe that no matter how old the bike is a good modern oil is better than the best old oils. I guess my question is, if I use a Classic multigrade and use the bike mainly for short regular runs, how often should I change it to ensure that high loaded parts like cams and followers are not worn badly on start ups. I'm using Duckham's 20:50 which has a very high zinc content and will change the oil at around 1500 miles or once a year. It's contentious I know but I reckon for the relatively low cost for 1.5 l of 10:30 fully synthetic I should use it?
I also believe that the oil should be high detergent to make sure that microscopic sludge particles get thrown out with the dirty oil. The sludge traps such as on the Honda and old Triumphs don't take any sludge out with modern fuels and detergent oils in fact they don't seem to do anything.