Post by: Craizeehair on February 25, 2026, 09:47:19 PM
I have also tried to do some reading around the correct oil for these bikes / clutches and a mineral oil, 10w40 JASO with zinc if possible seems to cover most bases.
I did find an old post by HondaMan on the subject stating that these bikes should in fact use 20w50 and the 10w40 that was actually listed by Honda was a mistake, does anyone know if this is correct, could he be talking about when using in warmer climates than ours, I am having trouble getting out of this particular rabbit hole.
Any suggestions for a tried and tested oil that suits these bikes and helps with clutch drag would be very much appreciated!
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Post by: Yetanotherbike on February 26, 2026, 08:14:11 AM
https://bobistheoilguy.com/forums/
Post by: TrickyMicky on February 26, 2026, 10:23:12 AM
Post by: Craizeehair on February 26, 2026, 01:25:27 PM
As a relative beginner to all things related, I tried to research and find the causes and as I came across HondaMans post, stating the very first thing to do is check you have the correct oil and 10w40 was a mistake my Honda, that was enough to start the rabbit hole.
I also found that he states that the clutch assembly diagrams from the Honda fiches in their books are incorrect as well.
Now I’m so confused I don’t know where to start
The clutch itself is very stiff and hard to pull, so that will need some sorting. I hope to get this issue sorted and ready for the years riding but feel a little lost.
Hence the question about the oil from the UK members, just to satisfy myself before moving on to the potential clutch issues.
Post by: TrickyMicky on February 26, 2026, 02:26:13 PM
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.
Post by: K2-K6 on February 26, 2026, 03:01:27 PM
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!!
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.
Post by: K2-K6 on February 26, 2026, 03:11:21 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.
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.
Post by: Priddy on March 07, 2026, 06:05:20 PM
Post by: Bryanj on March 07, 2026, 07:54:50 PM
Post by: Skoti on March 07, 2026, 09:13:55 PM
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?
Post by: Craizeehair on March 07, 2026, 09:48:15 PM
His advice of check the oil first had me thinking if I have the right oil, I guess it’s a process of elimination but just trying to figure out where to start first?
Post by: McCabe-Thiele (Ted) on March 07, 2026, 11:17:01 PM
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?
When I was 15 (1963) I used to work on a petrol forecourt at weekends, one of my jobs on Sundays was to fill up the 1 pint and 0ne quart Castrol Oil Glass Jars that had a wire clamp lid up to the measure line with either Castrol L, XL or XXL although the name rings a distant bell I had to Google Castrolite to find it was an early Multigrade oil launched in 1948. I do remember Castrol GTX coming onto the market, at that time the forecourt sold more Duckhams 20/50, the cheaper alternative was either Redex Motor Oil or the locally produced Silkolene Oil that was said to be recycled oil made in the Belper Refinery.
Post by: Bryanj on March 08, 2026, 01:14:44 AM
Post by: SteveD CB500K0 on March 13, 2026, 08:12:26 PM
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Post by: Martin6 on July 07, 2026, 11:00:35 AM
I'd be happy to use 10W40 though. I think the best advice is don't exceed the mileage between changes.
Post by: Alimacdee2 on July 07, 2026, 08:40:26 PM
Post by: McCabe-Thiele (Ted) on July 07, 2026, 10:52:40 PM
A similay grade synthetic oil would have less resistance but increased flow rate.
The corollary of that is a drop in pressure soley by the use of a thinner oil actually increases the amount of lubrication.
My memory of Jaguar & Mercedes engines from the 1960's is that they ran on much lower oil pressures than the likes of BL.
When I ran an old Nissan Diesel 2.7 4 cylinder 4x4 the Nissan Owners Club were of the view that Mineral oil was best, I found that using fully synthetic engine oil of the same grade (10W/40)actually made the engine much quiter without any noticable loss of oil between services.
Post by: Martin6 on July 07, 2026, 11:36:49 PM
Honda itself said 20W50 is fine.
Post by: McCabe-Thiele (Ted) on July 08, 2026, 08:06:57 AM
Regarding the Honda, I agree with that, Ted. But on a well used oil pump and main bearings (and other tollerances), does that still hold true?
Honda itself said 20W50 is fine.
I don't know the answer to that question, fluid dynamics was never my strong point, I've had lecturers tell me to think of pressure as Voltage, flow rated as Amps, resistance is a tricky one that has many variables depending on the fluid involved. A thin film of good oil might be better than a thicker film of mineral oil.
Interestingly the development of synthetic oil was initially based on longer service intervals. One issue was the build up of acidic waste in the oil from sulphates iirc. The game changer was to add micronised chalk to the oil to neutralise the acid build up.
Post by: florence on July 08, 2026, 09:21:14 AM
Post by: McCabe-Thiele (Ted) on July 08, 2026, 09:33:16 AM
Should we not also consider the temperature of the oil and it's viscosity at start up, a critical time for the engine. I certainly would not want 20w50 in a cold engine in the winter. (Perhaps you don't ride in winter?)
At one time you could buy pre-oilers for many cars, a servo pumped oil giving full oil pressure before you started the engine.
Post by: Bryanj on July 08, 2026, 02:03:58 PM
Post by: Alimacdee2 on July 08, 2026, 04:03:40 PM
Should we not also consider the temperature of the oil and it's viscosity at start up, a critical time for the engine. I certainly would not want 20w50 in a cold engine in the winter. (Perhaps you don't ride in winter?)
don’t think it’s a concern in the UK, even the North of Scotland it’s rarely that cold, I usually drive all year round weather permitting when it’s dry, I keep bikes heated in winter to prevent condensation &oxidation forming on engine casings when temperatures drops
I’ve also previously found lower viscosity semi synth and full synthetic oils exacerbate problems with leaky cylinder head pucks, threads, and exhaust valve stems on the early cb750 on an older rebuild, and.. now that I’ve heard it, probably due to fluid dynamics and increased flow rates