Lawn mower engines are precision engineered, and as such need to be maintained properly for them to work as they should.
It goes without saying that part of the maintenance programme for a lawn mower engine is changing the oil at least once per year and making sure that the replacement oil is the right type for that particular make and model of mower.
Generally speaking, lawn mower petrol engines fall into two categories. 2 stroke engines and 4 stroke engines. Both these engine types use very different types of oils, which can not be mixed or swapped around.
Always consult the owner's manual to check whether the mower engine is 2 stroke or 4 stroke.
Both motorcycle engines and some lawn mower engines are classified as 2 stroke. This class of engine uses only 2 movements for one piston cycle, and has an oil that is added to the petrol in the petrol tank. The subsequent fuel/oil mixture is then burnt in the combustion chamber. Because of the way it is used, 2 stroke oil has different additives in it which are not present in car engine oil which is a type of 4 stroke oil. If 4 stroke was to be used in a 2 stroke engine the results would be catastrophic. The engine would be a complete write off. However a 2 stroke lawn mower engine is able to use any 2 stroke motor cycle engine oil with no damage being caused or side effects.
The vast majority of lawn mowers these days have 4 stroke engines. Basically, a modern 4 stroke lawn mower engine works along the same lines as a modern car engine which is also 4 stroke. However, there will be certain differences based upon size of the engine, as well as the ways in which they are cooled.
Again check in the owner's manual which oil is best suited or reccommended for that particular make and model of lawn mower. Whenever possible purchase the manufacturer's reccommended brand (usually their own brand), or a purpose made 4 stroke lawnmower engine oil. Having said that, specific lawn mower engine oil is becoming harder to find these days.
Lawn mower engines are able to operate with automotive oil in them, but it really depends on which oil is used. When looking for engine oil be sure to look out for additives. Many automotive oils contain ethanol in their composition or blend. While its's fine to use ethanol in car engines, it will cause serious damage to a lawn mower engine on an exponential scale, that is, ethanol's damage will start off almost negligeable and then compound, leaving the lawn mower engine seriously damaged and beyond repair. There are other additives that have similar effects and a little bit of research will go a long way, and is highly advised.
Usually oils are classified by their viscosity, and are graded by the Society of Automotive Engineers, hence SAE
The vast majority of motor oils are synthetic or blended oils and instead of the usual notation of SAE 30 for example, something such as 10W-30 will be on the bottle or tin. It can be utterly bewildering.
When an oil is denoted as 10W-30, what it is saying is the oil behaves as if it has a viscosity of 10 in lower temperatures and at higher temperatures it acts as if it has a viscosity of 30. The W stands for winter.
If lawn mower engine is unavailable or is just too expensive, which it can be. There is now real harm in using certain grades automotive oils. If possible steer clear of oils denoted SAE 5 to SAE 20 as they will cause damage. The best range of automotive oils to use in a lawn mower engine would be anything from an SAE 30 to SAE 40 of the non synthetic oils these oils are probably closest to suitable lawn mower engine oils.
So what of the synthetic oils? Again the best of these oils to use in a lawn mower engine is something between 10W-30 to 10W-40. However, read the label and look out for any additives such as ethanol which will ruin the engine in a relatively short time.
Lawn mower engines are different to both motorcycle and car engines, even though they are classified in the same way, as either being 2 stroke or 4 stroke.
A 2 stroke mower engine however, can use any 2 stroke motorcycle engine oil with absolutely no ill effect, but will suffer irreparable damage if using a 4 stroke automotive oil of any kind.
While a 4 stroke mower engine can use certain 4 stroke automotive oils, both normal and synthetic, care has to be taken in choosing the right one. A simple rule of thumb is to be aware of additives and use an oil with a designation of SAE 30 or SAE 40. Or with synthetic oils, 10W-30 or 10W-40 can be used.
It cannot be stressed enough, but always follow the advice and reccommendations in the User's manual.