SOHC.co.uk Forums > CB750
What engine oil
Alimacdee2:
I’m now using a 20/50 mineral oil, full circle really! I think it’s probably more appropriate for these older engines, these are hot running engines 20/50 at its hottest i see drops to 60psi on my bike, a 10/40 or 10/30 synthetic non synthetic oil would reduce oil pressure, to main bearings, the engineers that designed these must have calculated oil pressure according to engine clearances to match Oil and viscosity back then.
McCabe-Thiele (Ted):
I suspect that its complicated in that, as viscocity increases the flow rate decreases.
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.
Martin6:
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.
McCabe-Thiele (Ted):
--- Quote from: Martin6 on July 07, 2026, 11:36:49 PM ---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.
--- End quote ---
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.
florence:
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?)
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version