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[Presto]

Sorry! I meant Rattlebattle's choice!!

[zonggong]

Presto ... I just wrote an detailed response to your query only to have it disappear into the ether when I posted it. I'll try again later

[Leon Novello]

Presto: Combining a spark plug thread with an oil topic can send the board into meltdown.

[zonggong]

My Cortina let go on the motorway at 70 mph. The cabin filled with white/blue smoke that billowed everywhere. I drove it home like that. The #3 piston had a hole melted in it and molten metal had run down around the rings.
The SAE engine oil grades are measured at 100 C (originally was 212 F) as far as I know, due to the approximate cooling water temperature in a water-cooled engine, and, at the time these were established (in the 30s), was thought to be about the temperature that the oil ‘saw’. The SAE 40 grade spans about 13.0 to 16.5 cSt at 100 C and the SAE grade spans about 17.0 cSt to 21.5 cSt at 100C. This is the same for multigrade or monograde oils although the multigrade oils are often at the lower end of the range (easier to meet their multigrade requirements). If we take the midpoints there is about 6 cSt at 100C difference between the two. This is a reasonable amount at this temperature. If we take Presto’s point about differences in ambient temperature we can expect that with an air-cooled engine the differences in oil temperature could be significant (as long as there wasn’t extended periods of idling). A good quality 10W – 40 engine oil will have a viscosity of around 100 cSt at 40 C and 15 cSt at 100C, whilst a good quality 15W – 50 will have a viscosity of around 135 cSt at 40 C and 20 cSt at 100 C. These oils have a viscosity index (VI – an arbitrary index of the oil’s viscosity variation with temperature) of about 150 so if we were to graph them we would have parallel lines (need to use log paper because it is not linear). We would see that the 6 cSt at 100C becomes larger as the temperature reduces.
With the unit construction engine the engine oil also lubricates the gearbox. Multigrade oils generally use an additive, called viscosity index improver that is added to the base oil to improve the VI and make it a multigrade. This additive is a large polymer that expands when hot and curls up when cold. The shearing effect of gears can ‘chop up’ the VI improver and reduce the overall viscosity over time. The lower viscosity grades of oil now on the market e.g. 0W - 30 and so on, are specified by manufacturers for fuel efficiency. This is fine as long as the surface machining is really good and the tolerances excellent.
From my experience an oil viscosity of around 15cSt at 100 C is fine for a piston and liner so Rattlebattle’s selection of a 10W – 40 would be OK. I think at worst there could be some more oil consumed (gets past the rings and burns) or maybe leaks out some seals. It will certainly get pumped around the oil circuit more quickly when cold.
I have been using Penrite MC4-ST 15W – 50 in my B5 and it seems OK. This choice was more about availability than anything else.
I agree with Leon that it may be out of place with the initial topic but the discussion developed into one of engine temperature. Hope we haven’t caused any meltdowns.

[Presto]

Thanks zonggong for taking the time and trouble! Now I'll try to absorb what you've told us

[PeteF]

"It will certainly get pumped around the oil circuit more quickly when cold"
Surely that depends on the type of pump. If it were an impellor type then you have a point but a positive displacement pump would be no different? Oil, as I understand it is basically non compressible.

[Presto]

That may be true Pete - but a 'thicker' oil will put more stress on oil pumps -n indeed too much stress in some cases. So maybe 'more easily' instead of 'more quickly' will apply in that case.

[Rattlebattle]

Sorry to all; I've changed the original thread. This never happens on this bulletin board does it so I've given myself a blo*dy good thrashing......Recovered now and bruises turning multi-coloured. Thanks zonggong for the concise explanation. I have found that if I free the clutch off every day it doesn't stick, but if left for a week or so it takes some doing to free off. By contrast the clutch on my old Tiger 100 always frees off easily. That has 20 grade oil in the chaincase. This is what set me wondering whether in the UK my C5 would be better off on a 10W/40 (or 10W/50). I also have a Hinckley Triumph twin. That uses 10W/40 oil (or 10W/50) of the same JASO spec (MA), but to SH or higher. (The C5 is SL or higher). Apart from the fact that the Triumph is partially oil-cooled, it has the same basic wet sump lubrication system ie a single oil lubricates and cools the engine, clutch and gearbox. Triumph now recommends Castrol Power 1 oil (they used to recommend Mobil 1). As far as I can see there is no compelling reason why I shouldn't use this easily available oil in my C5. The fact is the engine runs really cool in that bike - by contrast the Trumpet runs very hot. My guess is that RE only recommends Motul because they have some sort of deal - like Triumph they could just as easily switch to another supplier. My C5 engine is quiet and uses no oil. It didn't rattle and more when I bought it with the 10/40 in it. The only thing I wonder about is that RE specify an ester based oil. so, zonggong, is that anyhting special or is it really just a more specific way of saying a synthetic oil? FWIW I've got enough oil for a couple of changes so any experiment will take a while anyway - it's too expensive not to use.

[zonggong]

My handbook doesn't specify an oil by name .. only by spec. For our purposes oil is considered non-compressible. In considering the flow of oil around the engine we need to consider the whole oil delivery system, not just the pump. The oil delivery system includes pipes, filter etc which have their own pressure drop associated. For example, there is a relationship between pipe diameter, length and viscosity which can calculate the pressure drop in the pipe and the resultant oil flow. This would show that 10W-40 will flow more easily than 15W-50 , especially on start-up when the oil is cold and the viscosity much higher than at operating temperature.

[zonggong]

Hi again ... I missed Rattlebattle's last query. I am surprised that the oil spec for REs says 15W-50 API SL grade JASO MA, Ester-semi-synthetic oil. I suspect this may be what their preferred oil supplier told them to put . Ester can be a small constituent of the base oil as far as I know. ... bit like an additive For example, some viscosity grades of a very well known synthetic engine oil is made up of Polyalphaolefin (PAO), Alkalayted Napthalene (AN) and Ester. The PAO makes up the majority of the base oil. Incidentally, this is chemically derived from Ethane, C2H4, to make an artificial hydrocarbon chain. The PAO is very pure and will not mix easily with some additives and also could potentially harden seals (common Buna N type). Ester is aromatic and its addition as a component of the base oil allows additives to be more easily mixed and also allows for a little seal swell to couteract the hardening/shrinking effect of the PAO. If RE are referring to ester as a component then I think most synthetic oils will have something similar although they only say semi-synthetic, which is very general and does not specify the percentage of synthetic/ mineral. I don't think a semi-synthetic oil would normally require ester because the mineral oil component would be enough to provide miscibility and seal swell. There are many thousands of esters and only some are suitable for lubrication. A few years ago the oil industry accepted a newer definition of synthetic which included heavily hydrotreated base oils known as Group III . As far as I'm aware there is a lot of this available and it is less expensive for the oil marketers than PAO. PAO is called Group IV and other synthetic oils (like Poly Alkalene Glycol) are called Group V.