Multigrade or single grade oil?

Previously wrote:

Chatting to an alledged expert dealer earlier last year, I asked why he insisted that owners of newly built Commandos must use SAE40 grade engine oil, else the dealers warranty would be invalid. He replied, "Because, when hot, SAE20/50 only produces 45lb/sq.in, where as SAE40 produces 70lb/sq.in".

When I mentioned that the bikes oil pressure relief valve is set to open at 45lb/sq.in and therefore any oil pressure above this is redundant, the dealer replied, "No it dosen't", relatingto the relief valve opening pressure. I checked the workshop manual, the relief valve opens at 45lb/sq.in. Furthermore, I can only assume that he measured the oil pressure before the relief valve, some expert.

My bike's big ends have done over 100,000 miles before requiring a regrind - on 20/50.

Hi Simon,

I don't see how you can measure the pressure before the relief valve, ie nearer the pump because as soon as the valve opens the pressure in the whole system will drop to 45PSI ( you could remove the PRV and measure the uncontrolled pressure, not sure what that would prove) IMO 45psi is plenty of pressure for an engine of our design. The important thing is that there is some oil flowing. When I was a lad there were plenty of Anglias and Minis running around with their carlos fandango oil pressure gauges reading less than 15 psi when hot. Don't recall seeing many of them siezed up on the side of the road. The current going rate for Morris's SAE40 is about £18, their 20-50 is about £10 for 5 litres. Judging by how much of the stuff ends up in my chaincase, on my drive or getting by my rings, I'll go for the modern option and save myself some money

Previously wrote:

Chatting to an alledged expert dealer earlier last year, I asked why he insisted that owners of newly built Commandos must use SAE40 grade engine oil, else the dealers warranty would be invalid. He replied, "Because, when hot, SAE20/50 only produces 45lb/sq.in, where as SAE40 produces 70lb/sq.in".

When I mentioned that the bikes oil pressure relief valve is set to open at 45lb/sq.in and therefore any oil pressure above this is redundant, the dealer replied, "No it dosen't", relatingto the relief valve opening pressure. I checked the workshop manual, the relief valve opens at 45lb/sq.in. Furthermore, I can only assume that he measured the oil pressure before the relief valve, some expert.

My bike's big ends have done over 100,000 miles before requiring a regrind - on 20/50.

I run my 750 and 850 commandos on Silkolene monograde 50( detergent )oil . I also use the same in my AJS 350 single . I have done so for several years and changed it at regular intervals of 1000 miles or per year , whichever comes soonest . ( my annual mileage is spread over 5 bikes ,and isabout 1000 miles per bike peryear . ) The 750 has an oil gauge fitted and fed from the rocker feed outlet at the base of the timing chest. As such the oil pressure is regulated by the relief valve . However i find that on start up i have 60 to 65 psi on the gauge which suggests the relief valve is incorrectly set .This drops to about 45 when the engine is warm and running at 3000 rpm . As the oil temperature rises further (vis after an hours running ) the pressure drops to about 35 psi , again at 3000 revs , but drops to 15 psi at tickover . I am happy with this arrangement , so will continue using the Silkolene 50 but its realy a matter of personal preference . If you go to the technical section of the web site under Commandos there is a thread on oils and pressures which makes interesting reading .

Here's that link to the info in the Technical Section. - Personally I've always used SAE40 Man and Boy after a recommendation by "a famous expert" - Must be made of money?

I use the multigrade option, as suggested by my local expert. Easier to get hold of and cheaper.

Modern high quality oil should be fine - 20w50 - mobil 1 seems a good bet, especially in cold weather.

Rob

Whenever you find two NOC members there are likely to be three opinions on oil! The Commando was launched with multigrade oil. So it must be alright.

Running it on a straight viscosity oil may involve more compromises than a suitable multigrade oil.

Would I use a fully sunthetic oil in a Commando? No. I would use a multigrade 20w50 traditional oil. I would preferably have a decent filter in the system too and then I would change the oil frequently.

..but they gave problems from new on mains, cams and followers so perhaps the then new multigrades weren't such a good idea ?Personally, after extensive scientific testing, I use mongrade 40.These tests include:-

1) sniffing it. Monograde smells more like oil for a proper motorcycle.2) Spilling it on the garage floor. It sticks like hell so it must do that in an engine as well.

3) Listening to it. I'm convinced that my engines sound quiter and smoother than when running with multigrade.

4) Most importantly, I'm happy with it and it fits in with a mindset that likes old fashioned things and is instinctively suspicious of anything new.

I wouldn't really expect problems with mains or big ends regardless of the type of oil (within reason). The major problem area with the Commando engine is the camshaft / follower interface and the quality of these has been so variable over the years that it's difficult to judge what effect oil may have had on premature wear.

What thisdealer is saying sounds a little bit 'iffy' to me.

I notice that several of the responses, from other owners, to your first message, have got the facts absolutely spot on. If you bought a new Commando in the early 70s it came filled up with an oil tank full ofmultigrade. Generally a 20W/50. The oil pump, when new, would pump out around 75psi on start up, which did indeed usually trigger the factory set pressure release valveto open at about 45psi.At this point, bear in mind, thatthe opening of this valve does not mean that the remainder of the circuit is operating at45psi. eg the left hand big end, being at the end of the feed is would probably register far less than thiswhereas a pressurizedrocker feed might be slightly above.

The late John Hudson had verystrong views onthe type ofoil he recommended for the road bikes that he tested and serviced. His favourite was Castrol GTX which by 1970 had been uprated from 15W40 to 15W/50. His reasoning being thatthis oilnow hadqualitiesmaking it almost unversalfor usein in nearly all areasin Norton twins. That is it could be used in the engine, gearbox, forks and primary drive. He said that the 15W rating made turning over a Commando engine, especially in winter, far easier and that once running this type of oil got round to lubricate the vital components of the enginemuch quicker than a monograde thus cutting wear down considerably. He also pointed out that on fast, long run a very hot monograde oil will not retain its lubricating properties as well as a multigrade.

On the old Norton website there were numerous articles confirming John's views and also pointing out how exceptionally good the modern synthetic multigrades are compared to almost all other oils available nowadays.

I know what I oil I put in my Norton engine and the reason why.

Phil Hannam

hi simon lisen to phill he's right I use Millers 20/50 in my Norton manxman650, And have no trouble and starts as easey as a 500 dommie 88 frist or second kick ?yours anna j dixon

It seems to me that the research and development carried out by oil comapnies over the yers will only have improved performance for all engine types. Personally I find that a semi-synthetic multigrade seems to be a good compromise between a modern oil and paying an extortionate price! I like the article on the following website; it seems to be well informed.

http://www.realclassic.co.uk/techfiles/oil030319.html

Howard Biddle

I believe the reason that mongrade oils were preferred for motorcycles in the early days of multigrade oil was the fact that the polymers in a multigrade would break down under the heat of an air cooled engine to quickly render a 20W50 into an SAE20.

Since then, polymer technology has advanced, such that a good quality multigrade will retain its viscosity characteristic for much longer than in those early days.

Synthetic oil employs a different technology and will retain its viscosity characteristic for an even longer period, and although it may at first appear expensive, you can go for longer between changes so there is little difference in price. To give an example of the capabilities of modern oil, my car uses a long life synthetic oil which costs about £60 per change. The car employs computer based condition monitoring to work out when an oil change is required. After 4 years and 40 thousand miles it still said it was not quite yet time for a change, but I erred on the side of caution and changed it anyway. Of course a big lazy water cooled engine is kinder on its oil than our air cooled twins, however considering the relatively low annual mileages most of us do on our bikes I would consider it well within the capabilities of a good quality synthetic oil to last for 2 years in an air cooled bike fitted with an oil filter.

As a poster on another Norton forum once said:

Some oil is better than no oil.

New oil is better than old oil.

To this I would add:

Expensive oil is cheaper than an engine rebuild.

Previously wrote:

I believe the reason that mongrade oils were preferred for motorcycles in the early days of multigrade oil was the fact that the polymers in a multigrade would break down under the heat of an air cooled engine to quickly render a 20W50 into an SAE20.

Since then, polymer technology has advanced, such that a good quality multigrade will retain its viscosity characteristic for much longer than in those early days.

Synthetic oil employs a different technology and will retain its viscosity characteristic for an even longer period, and although it may at first appear expensive, you can go for longer between changes so there is little difference in price. To give an example of the capabilities of modern oil, my car uses a long life synthetic oil which costs about £60 per change. The car employs computer based condition monitoring to work out when an oil change is required. After 4 years and 40 thousand miles it still said it was not quite yet time for a change, but I erred on the side of caution and changed it anyway. Of course a big lazy water cooled engine is kinder on its oil than our air cooled twins, however considering the relatively low annual mileages most of us do on our bikes I would consider it well within the capabilities of a good quality synthetic oil to last for 2 years in an air cooled bike fitted with an oil filter.

As a poster on another Norton forum once said:

Some oil is better than no oil.

New oil is better than old oil.

To this I would add:

Expensive oil is cheaper than an engine rebuild.

My preference is for a cheap(ish) multigrade that will only be in the bike for 1,500 miles and then replaced. Sadly the amount of oil that my Commando swallowed most of my oil didn't last between services. I think the bike had instituted a programme of rolling oil changes