Thanks for all three replies.
10w -40 is the answer. Excellent!
You may be puzzled by my question...I must stop asking stupid questions
But sometimes the oddball question gets the right answer straight off the bat!
My reasoning? Just making sure I'm on the right lines!
Thanks again, now I'm clear on which oil to use summer and winter.
I'm fascinated by the subject of engine oils and lubricants in general. It's tempting to think of oil, and engine oil in particular, in terms of how "thick" or "thin" it is and this simplistic view may not be too helpful when considering modern lubricants.
When I started "messing about" with cars in the mid '50s it was old and unroadworthy bangers which I ragged around the fields and dirt roads. Becuase they were wrecks before I got them, and I was abusing them something rotten, they often broke which caused me to learn how to repair and improvise - stood me in good stead as I later learned "properly" how to repair and maintain vehicles. Oil, when needed, was pinched from the machine shed which had mostly stuff like cultivators, mowing machines, sometimes a tractor, etc and I had no idea about oils so just took whatever was there. Sometimes this would be "winter" grade and marked 40W or summer 30W or even gear oil which was much thicker - no wonder the poor engines suffered problems. Then, when I went to college and started working in the garage trade I was exposed to the wonderful world of automotive lubricants in all their diversity. Until then It had all been quite simple and I'd learned that some oils were "thick" and others "thinner" too thick and the engine would struggle to even start, too thin and the ends would knock with clouds of blue smoke in the exhaust! Around that time Multigrade oils started appearing which put an end to having to run "winter" and "summer" oil. However it required a major rethink of how I looked at lubricants.
There are may aspects to lubricants but I'm only talking about viscosity here. If you think about a Mono grade - say SAE 30, which is very common in older air cooled engines and I still like to use in small air cooled lawnmower engines (getting hard to source though) then it's viscosity (how easily it flows) is going to be directly relative to how hot it is. The colder it is the "thicker" it will be and, conversely, the thinner it will be depending on how hot it is. This, pretty much linear, relationship - within limits - between temperature and viscosity (thickness) is very useful in air cooled engines, especially where they get used mainly in the warmer summer months and because a mono oil will be unlikely to "fail" within the operational temperature range for this application. You need to be a bit careful where a multigrade is specified for your lawn mower or other small air cooled engine not to let it get too hot as the polymers which make it a multigrade can break down catastrophically when too hot whereas a mono grade just gets linearly thinner with increasing temperature and you'll tend to let the machine rest when it starts burning the thin oil and smoking rather than the catastrophic failure a multigrade undergoes when it exceeds it's max temp range and breaks down invariably causing maybe big end and bore damage I find. Many small modern mower engines have aluminium cylinder bores where the rings run directly in the ally bore, If the oil breaks down the rings pick up in the bore and the result is very predictable. Most of these engines need as much help as they can get anyway as they have no filtration to keep the oil clean and people don't change the oil anything like as often as they should.
Ok, wee lecture about "small engines" delivered! So let's now have a think about modern multigrade oils. It's all very technical and you need to be a bit of a Chemist to understand it all - I don't pretend to understand a lot of it, especially regarding some of the more "exotic" additives - however the fascinating thing is how they don't behave like a mono grade oil. When looking at an oil for any engine then if it's a mono grade it will be spending most of it's operating life at the normal running temperature. Often around 80 to 100 degrees C but may be locally much higher. The bulk of the oil in the sump will be around that temperature though. So a mono grade oil will be chosen to be at optimum viscosity regarding bearing clearances etc, when at the normal operating engine temperature. This means there will be a measurable time of operation when the engine is outside that temperature and the oil will be too cold - so thick - or too hot - so too thin and so not providing optimal performance. A multigrade goes a long way towards mitigating this. It will be based around a quite low viscosity when cold thus helping the oil to circulate when the engine is first started. 10, 20, 30, 40 weight as not unusual depending on the manufacturer spec. Of late much lower specs of 5 and even 0 Weight are quite common. The advantage of this is that the oil can very quickly get to all parts of the engine where it's needed, especially from a cold start but also it cuts drag when the engine and oil is cold thus dramatically reducing frictional losses when cold. The second part of the spec numbers - so the 40 in 5W40 for instance - relates to how viscosity of the oil when up to operating temperature. Very important you understand that this is a measurement of viscosity relating to the oil when HOT. So the 5W relates to a cold oil and the 40 to a hot oil. You'd expect the oil to be thicker when cold and much thinner when hot but this oil has the flow characteristic of a 5 weight oil when that oil is cold and of a 40 weight oil when that oil is hot. People have a lot of trouble understanding this because it's a bit like comparing apples and bananas, ie, not like for like. Very basically, what this means is that quite quite a "thin" oil can be used as the base product giving all the advantages of a thin oil at cold start but, by including some clever chemistry, the oil can be stopped from thinning catastrophically when hot. This is done by using polymers (or so I think I understand) What? I hear you say. Well this was explained to me as imagining the oil contains shed loads of very little octopi! Yes, those long armed sea creatures. Imagine the sea is the base oil with all these creatures crammed in together. When they are cold they keep their arms wrapped tightly around themselves so they just float about in the water (base oil) and don't really restrict how the fluid flows. As they heat up they uncurl their legs so start to resist how the fluid flows and the hotter they get the more they unfold their arms. So the oil doesn't get "thinner" as it heats up. Pretty clever eh? I've found this works especially well in fluid cooled engines where cooling and internal temperatures are pretty constant when working within design limits. Air cooled engines are nothing like so kind to their lubricants as they are well know for suffering localized hot spots and the problem with this is that once you exceed the design limitation of a multigrade the Octopusses - oops Octopi - quickly give up and the lube reverts to being about as much use as water! Thus my liking for a mono grade in my wee air cooled horticultural engines.
I learned a lot about this when I ran my gardening squad. We ran a mixture of machinery from quite small domestic machines, for going round flower beds and into awkward access areas, through some medium size self propelled commercial grade rotary machines up to a couple of ride ons. One of the biggest problems I had was when I started someone new on the squad they would need to get "tuned in" to the machines. Invariably they would not be able to judge when a grass collection box was full. These machines were all rotary - I only had one cylinder mower for doing a couple of "ornamental" lawns at a church retirement home where they wanted the "stripped" look - and with the rotary machines it's very easy to just keep on cutting with the catcher full. Because the blade can't clear the cuttings it starts to bog down which makes the motor work much harder and, because these engunes are governor controlled, they just open up the throttle and keep going until so blocked up they stall. This results in a very hot engine indeed usually with whisps of smoke curling off the cylinder barrel! What I found was that often an engine running on a multigrade (as is manufacturer specified for the newer machines) would start burning oil and suffer reduced compression. When you opened them up the bores were scored, rings had lost their tension and other damage was evident. Engines being run on a monograde almost always survived abuse of this type and I'm sure it was because the multigrade would continue trying to compensate for the increase in temperature until it exceeded the design limit when it would very quickly revert to being so thin as to be useless whereas the monograde would just get gradually thinner and thinner, start burning it so smoking from the exhaust thus giving the operator warning that all was not well before catastrophic damage took place.
Think I'll stop here because I could go on aand I'm sure you're probably nodding off a bit by now?
