The Duckhams Oil Company were the inventors of the 20W50 multigrade oil which meant that motorist now didn't have to use a different oils according to the temperature or time of the year. This blend of oils was a revelation to the British automotive industry as it went a good deal of the way to compensate for the less than exact engineering tolerences and practices of the day. Your average 15 year old Morris Minor or brand new Triumph now sounded quieter for longer. (Still leaked though!) Then along came the Japanese with their clever metric measuring stuff and brand new machinery that didn't require Old Harrys' skills to compensate for clapped out 60 year old bits of kit. New machine tools meant the Japanese could consistantly work to tighter tolerences which naturally allowed for the use of thinner lubricants, hence the switch to grades such as 10W40. A thinner oil equals faster warm up and so less wear from cold start up; less stiction in moving parts so they can be made to move faster and lighter and cheaper. Thicker engine oil was no longer the way to go so the evocative odour of Castrol R became a faded memory where bikers gathered, and a jolly good thing that was too for any motorcycle engine rebuilder.
All this is just my version of the oil question as I hadn't previously come across anyone regularly using or advocating the use of 20W50 oil in any Japanese bike since the 1970's. Obviously there are some that did and still do and if that works for them, excellent, do what works best for you, I say.
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) Anyway it keeps the old brain active! Merry Christmas one and all, and a Happy and safe New Year. 
