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Question about petrol/gasoline engine oils
Halcyon:
I'm about to do a service on my petrol lawnmower next weekend which involves changing the engine oil.
The Honda manual specifies that it should be "4-stroke motor oil" with an API category of SJ (or better) and either 5W-30 or 10W-30 viscosity (I'm leaning towards using 5W since I live in an area with a relatively low ambient temperature throughout most of the year, compared to the suburbs of Sydney).
My local store has decent brand fully synthetic oil for $35 for 5 litres that exceeds the minimum SJ spec listed in the manual (this one is SN), but they also stock "speciality oils" for garden equipment and other small engines which are specifically labelled 4-stroke oils with a small 1 litre bottle costing $28 (which is also labelled "premium mineral oil").
What's the difference? Is there any reason I can't use the far cheaper oil designed for car engines, provided that the viscosity is the same as specified in the manual? Surely if it's good enough to go in a modern car, it's good enough for a lawnmower?
DavidAlfa:
I don't think there's any difference except the hole in your pocket, "Premium mineral oil"? Sounds like a rip-off.
I'd put the synthetic one without thinking twice, better oil, cheaper, it's no brainer!
floobydust:
They might have specified "4-stroke motor oil" to avoid confusion with "2-stroke motor oil" which if mixed up ruins an engine in no time.
The expensive specialty oil I believe is "non-detergent" oil. In the old days, the concern was about foaming with splash-lubricated small engines, and since they have no oil pump, something about sediment needing to settle down and collecting in the bottom. But they used metal additives like zinc which are out of favour and expensive nowadays.
There's some videos frothing up non-detergent oil and finding it did worse compared to modern oils, so I don't see an advantage in a modern engine.
Non-detergent oil has a place in transmissions and air compressors so it still exists.
I would just use 10W30 synthetic motor oil. Air-cooled engines generally run hotter than car engines, i.e. I find a 5W20 can get water thin when hot, supposedly great for fuel economy with lower frictional losses. Small engines warm up fast but I'm not sure what temps you consider cool. Here you can start and run a snowblower when it -20°C and 5W30 is good.
Honda lawnmowers I see the majority are using 10W30 motor oil, they have a plastic camshaft too so I would not use any old school oil.
Halcyon:
Thanks for your advice guys. I'll probably just stick with the synthetic 10W-30 as it's cheaper than buying small bottles of these so-called speciality oils. Both my mower and line trimmer take the same oil, so it's easier just to keep a larger bottle on-hand. Fuel economy doesn't matter to me, we're talking about a few dollars per year (if that). I just don't want to risk damaging the engine with the wrong oil. In terms of temperature, I'm unlikely to be mowing the lawn in anything under 10 degrees C anyway.
Benta:
There actually is a difference, which is well known to owners of vintage cars and motorcycles (I'm one).
The modern synthetic oils have additives to absorb particles (mainly soot) in the oil and carry them through the oil filter (or for fine particles to just keep them suspended in the oil).
The older minerals oils function differently, the idea is to deposit the particles in the bottom of the sump.
Now whether this difference matters to a garden machine is doubtful. Personally, I'd just buy the cheapest 5W-30 you can get. It's not likely that a garden machine will rack up 100k miles. :-DD
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