Quote from HondaMan
I suggest avoiding the Lucas stuff: it is very high detergent, which can dissolve the goo that holds the corks on the clutch plates. The first one to let go will the top plate, if your K1 has the top plate with slanted cork blocks. Otherwise, it will be either the top plate or bottom plate, if all plates are square-cut. Just my experience. STP Oil Treatment has a similar effect.
You might check the type of 20w50 oil: several years ago, the U.S. EPA removed most zinc from the oils unless they were for Diesels, Motorcycles, or racing. So, the Castrol folks were kind enough to make theirs the "4T" type for bikes, which is both low detergent and has a bit more zinc than, say, their automotive oil, or Valvoline. Even at this, the 4T oil does not shift as well as it did before this change. I have REAL good shifting back with the synthetics: Mobil 1 has been the smoothest at 15w50 weight, but likes to leak out the cam cover.
It made my tranny feel brand new, no neutrals, smooth as silk, in 500 miles with it. After 3 years, it started leaking at the cam cover, though.
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HondaMan Are you saying you were using the Mobil 1 synthetic 15w50 in the late 70 early 80 Honda fours? I have told people I used it and they tell me I am nuts. I know it turned an 81 wing into a new bike in 1995. Unfortunately I traded it for a newer model in a couple years so didn't have the experience with the leaks.