Engine Oil Viscosity...

[jmurray01]

My 100CC Mountfield mower says to use either SAE30 or 10W30, but the only oil I have for it is 10W40 (for 4 stroke garden machinery)...

Would that be alright to use ?

 

[yoster]

Probably fine, but I'll let the 'pros' confirm since I'm new here.

General rule about oil: The greater the difference between the two weights (in this case, 10 and 40), the less 'sheer resistance' the oil has. In other words, properties of the oil can break down faster. I'm not sure if this is an issue with these motors, but something to be aware of. If you do go with that large of a difference, see if you can get a synthetic. Greater sheer strength.

 

[Black Bart]

My mowers call for 10-30 but all I have ever used since new 26 years ago is 15-40 Rotella since it is a heavy duty truck oil and they don't have catalytic converters so Rotella has more ZDDP for greater engine protection.
Since your mower don't have a cat this oil would be ideal for it also.
Air cooled engines run very hot in the summer the 15-40 just gives a little more protection but I always allow mine to warm up before throwing against the Governor this lets the oil thin out some before putting it to work.

A few weeks ago I bought a new John Deere and the manual said use 10-30 or 15-40 care to guess what I'm going to use in it.:laughing:

 

[oldyellr]

Probably fine, but I'll let the 'pros' confirm since I'm new here.

General rule about oil: The greater the difference between the two weights (in this case, 10 and 40), the less 'sheer resistance' the oil has. In other words, properties of the oil can break down faster. I'm not sure if this is an issue with these motors, but something to be aware of. If you do go with that large of a difference, see if you can get a synthetic. Greater sheer strength.

Not true. I don't know exactly what you mean by "shear resistance" (or did you really mean to say "sheer resistance"?) but with multigrade oil, the higher number represents the viscosity at operating temperature, while the lower number is the viscosity at cold cranking temperature, something only of interest when starting a snowblower.

Be that as it may, to achieve the wider viscosity range, the oil either has to be synthetic or have more additives. It's probably distrust of all the additives that has given multigrade oil a bad name. Anyway, if you really want to read debates about oil, go to - Bob is the Oil Guy and read the forums. Lawnmowers are a relatively low stress, slow speed application and any motor oil will do, within reason. I remember once reading somewhere that 50-weight oil was the best for Briggs & Stratton engines, so I tried it. It worked just fine, but was a bitch to pull-start in the late fall when it was starting to get cold.

 

[yoster]

Not true. I don't know exactly what you mean by "shear resistance" (or did you really mean to say "sheer resistance"?) but with multigrade oil, the higher number represents the viscosity at operating temperature, while the lower number is the viscosity at cold cranking temperature, something only of interest when starting a snowblower.

Be that as it may, to achieve the wider viscosity range, the oil either has to be synthetic or have more additives. It's probably distrust of all the additives that has given multigrade oil a bad name. Anyway, if you really want to read debates about oil, go to - Bob is the Oil Guy and read the forums. Lawnmowers are a relatively low stress, slow speed application and any motor oil will do, within reason. I remember once reading somewhere that 50-weight oil was the best for Briggs & Stratton engines, so I tried it. It worked just fine, but was a bitch to pull-start in the late fall when it was starting to get cold.

Gotta love a post that starts with 'not true.' Well, I don't want to start some big oil discussion, but if you want to know more you can read for yourself at many oil forums, or bob is the oil guy, etc. I'm not talking about what the lower and higher numbers mean, in fact that's not all what I said, I'm referring to the breaks between the numbers and what impact that can have (outside of what the numbers themselves mean). And yes I meant shear resistance.

Did a quick google search on the topic, you can read for yourself here:
Motor Oil Viscosity Grades Explained in Layman's Terms
Scroll down to "Drawbacks of Viscosity Improving additives" - basically, the larger break between the two weights, the more additives required. Take that in conjunction with the information in that URL and you'll see what I'm getting at.

 

[jmurray01]

So basically 10W40 is fine!

My engine doesn't have a throttle, however I will let it warm up prior to putting it under any stress (I.E Mowing grass!).

 

[yoster]

I think you misread the information..

A few quotes from others that have discussed this before:

"For several years the number one recommendation of Briggs & Stratton has been synthetic 5W30 motor oil. They sell such an oil in SL formulation under their own brand name. Honda has recommended 10W30 oil in their engines for many years, with no mention of synthetic. B&S says that use of nonsynthetic multigrades in their engines might result in excessive consumption in very hot temperatures. They recommend 5W30 synthetic for temperatures from -20 to +120."

"Looking at the owners manual for my Tecumseh LV series engine, it says in their oil usage chart:

SAE 30W - For temps 32*F to 100*+F

SAE 5W-30 or SAE 10W - For temps -20*F to 40*F

SAE 0W-30 - For temps below -20*F to 40*F

Also states:

Using multigrade oil above 32*F will increase oil consumption. SAE 10W-40 is NOT recommended."

Basically, multigrade (especially 10w40 with all the additives it has..) oils are typically not recommended for air-cooled lawnmower engines. There is special multigrade 'for lawn equipment' oils out there that do NOT have these additives.. those are ok (you can tell also by looking at the oil.. it has a much lighter, almost clear color to it), though I think those are all 10-30w... not 10-40w. Single-base oils (like SAE30) also don't have these additives. Synthetic oils, while they have additives, hold up MUCH better in hotter temperatures, thus the reason why a multigrade synthetic oil is generally considered fine to use in these applications.

If anything 15-40w would be better than 10-40. 15-40 is a 2.6 factor difference.. while 10-40 is a factor of 4 (larger factor, more additives required, and a larger, negative, impact on shear strength as temperatures increase).

Of course you'll find the guy that says "I've used ____ for ___ years with no problems!" Probably true! Just want to point out the information and let you decide from there

 

[oldyellr]

So basically 10W40 is fine!

My engine doesn't have a throttle, however I will let it warm up prior to putting it under any stress (I.E Mowing grass!).

It shouldn't need warming up more than half a minute, unless it's really cold up there in Scotland (you did say elsewhere that your grass hasn't grown tall enough yet). But you're right, you don't want to put t it under load before the oil has had a chance to get to all the bearing surfaces.

And what jmurray01 indicated with the link he provided is accurate. While additives improve the viscosity index, they are what "wears out" in an oil, so it degrades with time and temperature. Synthetic oils achieve this improvement on their own, but unless it's a fetish for you, they're overkill for lawnmowers.

 

[jmurray01]

I don't know if this helps, but the 10W40 oil I have is almost as clear as water, so does that mean it has little additives in it and is suitable ?

 

[yoster]

I can't accurately answer that unfortunately. Unless it says it's formulated for garden equipment, I'd probably say no.

Just go down to the store and buy some regular ol' SAE30 and be done with it! $2.99 at Walmart lol!