Motorcycle Repair: best motoroil for cb360

Question
QUESTION: dear Bill,

need your opinion, what viscosity of oil is best for my cb360g from 1974. Service manual suggests 10w40, my old cycle expert stated that 20W50 is the best. Many guys on forums suggest to use 5W40 to avoid cam problem. Syntethic or general type?
The thing is that I can see many problems with the cam on the net because of insufficient lubrication and I faced with this problem also. I used 20W50 until now, but now my bike is getting oversize new pistons and rings (it seems that original was stucked earlier and because of that it had no enough compression).
So the question seems to be important and I would be gratefull if you can send me your opinion. Thank you in advance.

ANSWER: Karoly,  a Honda owner's manual will probably show 10-30 for regular use and maybe 20-50 for a very hot climate or continuous highway use on a long trip. Some of the bikes, back in the 1960-70s, just specified a straight weight 20wt or 30wt oil.

I always use 10-40 Honda m/c oil in our bikes, here in SoCal. Other than the camshaft bearing issues, the rest of the engine is on ball, roller, needle bearings which just need a continuous supply of clean oil to function as designed. Dirty oil, overheated oil or running the engine low on oil will cause upper end problems. The 10-40 oil will get to the cam bearings quicker than 20-50, especially in cooler weather conditions.  Today's motorcycle oils are formulated to work better than the oils of the 1970s, with additives that resist shearing that happens when oil is squeezed between transmission gears.

If your bike doesn't have any oil leaks, then you can use synthetic after you put about 500 miles/1000km on the new top end parts. Prolonged storage can create conditions where the piston rings can stick in the piston ring lands/grooves, reducing compression quite a bit, but be sure to check your valve seats and guide clearances, as well.

Bill Silver

---------- FOLLOW-UP ----------

QUESTION: H, Bill,
Thanks for your hints.
So you suggest to use traditional (not synthetic) oil for the first 1000km.
What does m/c mean (in 10-40 Honda m/c)?

Regards,
Karoly

Answer
Karoly, Yes, use conventional motorcycle (m/c) rated oil for the break-in period so the rings and pistons can establish a good wear pattern, then you can switch to synthetic oils. Motorcycle rated oil is formulated differently than automotive oil because of the demands of the transmission gear shearing effect.

Bill Silver