SOHC.co.uk Forums > CB750

What engine oil

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K2-K6:

--- Quote from: TrickyMicky on February 26, 2026, 10:23:12 AM ---I have no technical qualifications on this subject, only my memories. When I ran a 750 K2 in the 70's I used 20/50 oil. All the comics were singing the praises of the wonderful Duckhams "Q" multigrade, so, in it went.  My personal experience was "What a load of crap".  unless the bike was used on extremely long journeys, every time I took the dipstick out of the tank, the top of it was covered in white frothy sludge. I finished up using Shell Super Multigrade 20/50, and never had any problems at all regardless of type of usage. It also helped that my father worked in a Shell garage, say no more!!!  I carried on using this oil even when I got the 400, but then it changed to becoming Super Helix, also suitable for diesels. I then started using Motul 10/40 mineral oil, and have done ever since.  It's interesting to try to remember that there was life before the multigrade revolution, just over a year ago I did a cosmetic restoration on a 1966 CL160, and cast into the upper crankcase were the oil specifications, winter - straight 30, and summer - straight 40, and still they lasted!!

--- End quote ---

That's interesting, as I had exactly the same experience years ago. My dad loved that oil (BMC A&B series motors mostly) and suggested I tried it in my 750 K2, which produced the same result !  Like soap suds in a diswashing session   :) and always wondered if it was something peculiar, or that I had done.

Second point, I was "reading" crankcases over at DS open day, with some of the little twins stating "20 viscosity winter and 30 for summer use" clearly in the same vane.

Honda do publish data/information panels showing how different viscosity interact with ambient temperature for their engines.

Broadly speaking, these air-cooled engine from that era are true 30 viscosity hot/summer weather (typical UK) the 40 viscosity figure will allow for fuel contamination in service life, along with the oil shearing down below that 40 rating during its use, but still maintain the, hopefully, minimum requirement of target 30 viscosity.

K2-K6:

--- Quote from: TrickyMicky on February 26, 2026, 02:26:13 PM ---Engine oil in gearboxes is not the ideal solution, they perform much better on EP gear oils, that's why the older British bikes with seperate boxes had a superior gearchange, and it's not just us either, my mates in the 70's ran original type BMC Mini's and suffered from the same effects, cold engine:- nice smooth gearchange, but crap when hot.
  Regarding the clutch, 1st place to look is the cable, they are normally routed very close to the hot engine and suffer, I normally replace the cable about every 18 months/2 years, plus check if the handlebar lever is free in its mounting.

--- End quote ---

I question this view though, as it often comes up on here and out on the great interweb of facts.

These gearbox are nothing like a mini transmission and don't in any way follow any "transposed" logic of operational competence that could help.

The  mini is helical cut and uses synchromesh plus bulk rings which rely on friction to facilitate gear changing, that doesn't exit in straight cut dog gearboxes used in virtually all motorcycle.  There's very different causes of bulking change characteristics in car gearbox design that have no bearing in these Honda transmission.

I can give more detail if needed, that from speaking extensively to a Leyland transmission development engineer that worked on them.

In short, there's no relevance here.

Priddy:
 There is no end to this topic, regardless of vehicle it seems very much down to preference. I am running a new clutch and Castrol 20/50 Semi Synth and it is still a bit fickle, very grabby clutch and perversely if you 'blip it' on down changes (as I always have) it now crunches gears! considering Morris mineral oil (gold 20/50?) next time following advice from an 'I used to race these motors in a sidecar' bloke!

Bryanj:
All the years i worked for dealers we used 10w40 which at that time had zinc

Skoti:
Aye Bryan,

Same here, we always used 10W/40 Castrol, and during winter time 10W/30 Castrolite in Honda stepthru's etc.

Anyone else remember Castrolite oil from the Seventies?

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