Topic: What is the heaviest oil to use for CB750K1? Desert heat 110-120 in the summer

[PeWe]

Interesting read about oil viscosity, film strenght and pressure.
https://rennlist.com/forums/928-forum/1041459-engine-oil-and-last-century-porsche-road-engines-some-light-reading.html
It is the post 12-31-2017, 11:38 PM #1  by JET951 that is interesting

The details about film strenght on moving parts that have no oil pressure, only splashes of oil is like an old CB750 and other old CB's.

I start to understand why the K6 head I bought on eBay USA had such worn ex guides/valves when inlet looked like very low mile.
 Head in general in good shape, no bad threads.
Probably got too low viscosity oil like 10W-40, warm days and very hot rides. Lower quality oil loose it quicker, shorter change intervals.
It is the viscosity that is the point here, not synthetic vs mineral.
Synth withstand high temperature better and longer, though.

But, a better synth oil with lower viscosity can not replace a correct viscosity simple mineral oil which the thread was about. Experienced race shops write-up.

[Tracksnblades1]

Oil has evolved a lot faster than engine internal clearances have.

It's important to remember the old Honda air cooled engines have the same recommended clearances as the newer air/water cooled Honda's do.

While some lighter weight automotive oils formulated for stock (catalytic converter)  performing roller lifters may not contain the required additives for flat tappet engines, there are many light weight oils available that are more than adequate for all performance levels and temperatures. Some low viscosity diesel oil still offer sulfur and phosphorous additives for more extreme pressure ratings. 

For instance current Pro Stock technology from Vavoline Racing oil is their new 0w offering. 0 being *winter* temp J300 test temperature not weight. It pours like water.

Current NASCAR oil is Mobile 1, 5uperSynthetic 0W-30. Some racers are using both lower and higher viscousity oils too. Below the link indicates current oils may be 5w-20 for the roundy roundy guys.

And then the NitroMethane 70w racing oils. Formulated for the 8000+ Horsepower, restricted cubic inch engines, sporting internal components sizes of OEM offerings of 400 horsepower more/or less. The best explanation why 70w oil is required that I've heard is: Imagine an engine originally designed for around 350hp, bore it out to 500 cu in. Pump as much PSI the largest supercharger can make and pump that through the best flowing intake port you can make. Add one part of NitroMethane to one part air and you'll have nearly a 1000hp controlled explosion per cylinder slamming down on the stock size crankshaft journals, sending harmonics, vibrations, resonances, and torsional twists that can destroy the best crankshaft in seconds. Image all 8 hitting now on the same crank and 8 pistons and cylinder heads trying to comtain it all. Hell, he said, the whole d*m thing, cylinders , block, crankshaft bores, are huff and puffing, vibrating and resonating like a 16 year olds 15inch subwoofers hitting bass notes on a rap cd. If we could run tighter clearances without spinning bearings we would, our oil pumps require more horsepower than some cars have. We have to change oil after every pass...

Below are but a few of modern oils articles explaining important aspects of the same, furthering why modern manufacturers and racers have moved on...and 540Rat has been testing and publishing the results for years....

https://www.enginelabs.com/tech-stories/racing-oil-vs-street-oil-know-the-differences/

https://www.machinerylubrication.com/Read/30835/lubricant-film-strength

https://540ratblog.wordpress.com/2013/06/20/motor-oil-wear-test-ranking/

[Keith]

Its important to understand how multi grade oil works. A 10w30 oil is a 10 weight when cold. This allows oil to get to critical wear areas quickly. As the oil heats up, long chains of molecules form, causing the oil to thicken. As it cools, the molecules go back to their original state. The oil is not simply a mix of 10 weight oil and 30 weight oil. Mixing oils will defeat the way the oil is supposed to work.

[PeWe]

Maybe those thin oiled racer teams  do everything to win, teardown and repair after each race not a problem, right? Bearings are still shells in need of good pressure, not ballbearings?
Or the later design the Porsche thread post described, lower oil pressure needed to lubricate the rods. Still film strenght for surfaces that get splashes of oil.

The Porche post also notified the multigrade oil's base oil which the oil will end up in when additives have given up, a process that start very soon. Too thin base oil not good for those engines.  Also different engine designs of oil distribution to rods, later need lower oil pressure and therefore possible to use the modern 5-40 oils.

I' m convinced that same ideas are valid for a CB four too.

I have seen how quick a 0W-40 synth oil becomes much thinner when cold.

My car's oil feel bad after 9000km. 1.8 L engine with turbo, only 3.7 L oil. Not as good oil as my CB750 get  that get fresh oil more frequent I think the hot running turbo kills the small amount of oil quickly.

Multigrade oils that loose it also a reason for more modern car engines like BMW get expensive repairs when cam chain tensioner stop work due to the low viscocity and too low oil pressure. Chain can jump or snap. Audi engines similar expensive repairs. Sludge involved too.

I start to understand the very short oil change intervals Honda recommended, multigrade oil viscosity degradation.
Oil pressure gauge feels as a must where this process can be supervised. Pressure when really warm.

[ckahleer]

There was a guy on the KLR650 forum that had his oil analyzed at regular intervals between 2 oil changes. Oil degraded very fast during the first couple hundred miles, then more slowly after that. The oil change interval was 2,000 miles, but after only 200 miles the 10-40 oil was down to 10-30.

[PeWe]

He changed oil brand after that?

I'm sure that there are better or worse brands when it come to viscosity, how to fulfill specified values and how fast it will degrade.

If the gearbox feels smooth to shift and easy to find neutral when warm seems to be one way to verify the oil on a CB750 together with a pressure gauge.

I tried several oils on my K6 in the hunt for the right one. Both my CB750 have now relatively smooth gearboxes and no dragging clutches.