[ihusker]

I don't know if this has come up before, but I am wondering if I can use multigrade 20/50 instead of sae 30 oil in my old 4 stroke rotavators etc. I have quite a few bits of old machinery that were made before the advent of multigrade oil and they recommend 30 or 40 grade oil. In my very early motoring days the cars used to be the same, then came along multigrade 20/50 and we were all told this was the stuff to have as it was much better than the monograde stuff. More recently, when trying to recapture my long lost youth, I got a BMW R100Rs motor bike. This has a 1000cc aircooled flat twin engine and, if my memory serves me right, the recommended oil was 20/50. As sae 30 is not easy to come by in gallon cans and the small tins seem quite expensive, I am tempted to use 20/50 as it is available in all the Supermarkets. I wonder if any one knows the answer or has any views.

[Gwiz]

you can use 20w50, is the short answer. with most of the equipment ive come accross, the makers state sae30 as first choice, then 20w50 if you cant get the monograde.

[ihusker]

Thanks for your advice gwiz. I can't see any difference between an air cooled motor bike which recommends 20/50 and an air cooled horticultural engine which recommends sae 30. I'm just wondering why they still sell 30 grade oil for horticultural engines when 20/50 is cheaper. Is there some reason why 30 grade should be better. I wouldn't want to damage an engine which has lasted 50 years so far by using the wrong oil, but the 20/50 is far easier to come by and is a lot cheaper.

[Gwiz]

one of the biggest diferences between the motorbike/mower thought is that the motorbike will always have some type of oil pump within the engine. most of the basic engines we find on mowers/rotovators don't. what they do have is a splasher that flicks the oil about, hopefully in the right directions. this is one of the reasons you are not supposed to use a lot of these engines on a slope for any length of time.
you could say that thicker oils, when cold, won't be "flicked" around so well, and so you might get increased wear on start up.
also if the oil is too thin when hot, you might also get increased wear then as well.
oil viscosity can be very important.

i need a life. :roll:  

[ihusker]

Thanks gwiz. Never thought about the lack of oil pump. One thing that puzzles me. My understanding of multigrade oil is that the first number is the viscosity of the oil when cold and the second number is when the oil is hot. I don't know if this is true or not. If it is true, then, comparing sae 30 with 20/50, I would have thought that the 20/50 would be thinner when cold and slosh around more and when hot be thicker and give more lubrication than the 30, or am I missing something here.

[Gwiz]

this explains all, and gives a better alternative to 20W50
http://www.valvoline.com/carcare/articleviewer.asp?pg=ccr20040601ov
having read through it, it seems that 10W40 would suit your needs much better, i hadn't thought of that.

[ihusker]

Thanks gwiz. That link makes most interesting reading. You're right, it does seem that 10/40 is a better alternative to 20/50 and also to the original sae 30 - until you read the last bit where it says to use the manufacturers recommended grade. I suppose they have to cover themselves, but equally, when these engines were made. multigrade oil wasn't around, so I think, on balance, I will try 10/40 and keep my fingers crossed. Thanks again, gwiz, for your observations and input.

[Gwiz]

happy to help. if i can.

[deneview]

i use 10/30 or 10/40 in the lawn mowers , tractor mowers and rotavator, in my understanding this would be better than straight 30, as thinner when cold starting, but this is probably the cheapest oil anyway. only disadvantage with a thinner oil is it can find leaks through old gaskets, but amount of seepage prob minimal anyway.

[xs1.1]

Just read this link following a similar problem, a couple of things I found out:
Kohler only recommend using 10/30 and 10/40 in their engines below freezing, stating that above freezing the thinner oils tend to burn too easily and coke up the chamber so advise SAE30. If that is all there is to it I would probably live with topping up regularly and occasionally giving the head a clean out.

The issue with the feed is probably the key thing though, as previously stated: modern lubricants are made with the assumption that the oil will be fed by positive pressure where it is needed.  Static engines just thrash it around inside in the hope it finds its way by gravity or centrifugal force into the oil holes that feed the big ends.  OK then perhaps to go a bit thinner, but heavier oil may not find it's way in so quickly so you could have it running dry at start up.  

Another issue is that modern engines force the oil through filters, so is formulated to keep contaminants in suspension as long as possible until the filter can pull them out.  Engines that don't have filters need an Oil that allows the sludge to drop out and settle in the sump, rather than carry on transporting it around.    

The properly formulated SAE30 mower type oil is therefore the best option in the long run, I found a couple of suppliers searching for oils for Heritage or agricultural machines, Morris Lubricants sell a range of SAE30 oils for Horticulture or Agricultural applications, designed to carry on protecting from corrosion even if laid up for months as these machines tend to be.  http://www.morrislubricants.co.uk/ .