Though it’s usually stated what type you should use in the manual, it’s worth knowing a bit of background on lawn mower oil so that you understand how it works, and if alternative or substitute oils will be of benefit to the performance of your lawn mower.
The oil you choose will in part depend on the climate in your part of the world, as there are oils that perform better at different temperatures. For example when/if the outside temperature is below 5 degrees Celsius then an oil labelled 5W – 30 will perform best, as this will be formulated for use in cold conditions. For similar reasons warmer temperatures would be better suited to an oil labeled 10W – 30.
Type of Mower
You’ll need to change the oil in your lawn mower more frequently the more prone it is to contamination. This will be determined largely by the type of mower you have and how you use it. A rotary mower that isn’t cleaned regularly will accumulate a lot of grass cuttings and other grime on the underside of the deck, which in time can find its way into the oil sump. This is just one of the many reasons why cleaning your mower regularly, ideally after every use, is so important. Whilst a ride on mower may be slightly more immune to this problem given that everything is generally better separated and covered, it’s still good practice to adopt the same philosophy. Using higher quality oils such as those marked SG, SF, SH, or SJ are best used to avoid contamination becoming a problem too soon after an oil change.
Frequency of Use
For the average gardener, manufacturer recommended lawn mower oil is usually more than adequate as it will be designed for the mower to be used perhaps once or twice a month for half of the year. If for whatever reason you will be using your mower a lot more frequently than this than it would be a good idea to choose an oil that offers the benefit of allowing the engine to start easily, and also isn’t consumed too quickly by the engine. Oil marked SAE-30 will best fit this purpose, and although it may be slightly more expensive than other oils, it also has the added benefit of working in a range of temperatures, meaning that you can keep mowing in all weathers and at all times of year.
If your lawn has thick and tough vegetation then this, like any other other factor that increases the work that the mower has to perform, will mean that the engine burns more oil. So in a similar manner to the above scenario of increased frequency of mowing, an oil such as SAE-30 or 5W – 30 should be used as again these are less quickly consumed than other oils. You might pay a little extra for the privilege, however this cost will quickly be written off when you find the oil is lasting much better than the cheaper alternative.