15w40 is a multi-grade oil; which means that it meets the standards for both 15w and 40w at their respective temperatures. Multi-grade oils were developed to avoid the need to change oil from winter and summer. Viscosity means how thick or fluid the oil is, because in cold temperatures oil becomes thicker and more difficult to use. Before multi-grade oils were introduced, engine owners would have to use thinner oils in the winter to ensure the engine could still be used, and thicker oils in the summer. The lower the ‘W’ number, which stands for ‘Winter’, the better the oil performs in cold temperatures.
The 40 refers to its viscosity limit at 100 degrees Celsius (water’s boiling point), or the engines operating point. The numbers indicate that the oil has met certain specifications in recent tests, and these are performed by API (American Petroleum Institute) or ACEA (the European equivalent) so your bottle of oil should be marked with one of these. This limit is the same for all oils marked with that respective number.
There has been research exploring the idea that oil viscosity affects fuel economy, and its generally believed that thinner oil leads to better fuel economy. A Chevron Oronite study in 2009 entitled “The Lubricant Contribution to Improved Fuel Economy in Heavy Duty Deisel Engines” found that, for Semis, switching away from 15w40 oil to 10w30 could save 1% on your fuel economy, though this research may prove negligible to anyone who’s not putting in the miles driving across the country.
15w40 oil’s use was most prevalent for cars in the 80s and 90s, as well as for use in lawnmowers, farm equipment and other large diesel engines. The wide gap between the 15 and the 40 means that it will make for a much quicker start in older engines than other oils traditionally used. Most oils these days carry specifications for both petrol and diesel engines, so check the individual packaging for that specific oil’s specification. If you’re unsure what oil your engine requires, check the handbook for details.
There are mineral and synthetic oils, and you need to be aware that ‘synthetic blend’ or semi-synthetic may not be what you’re looking for if you want a synthetic oil, so make sure you read the bottle properly. You can expect to pay $12-15 per gallon of oil for a universal oil you can pick up at the store, such as this Shell Rotella 15W-40 engine oil at WalMart [link: https://www.walmart.com/ip/Shell-Rotella-T4-15W-40-Heavy-Duty-Diesel-Oil-1-gallon/181236405]. You can, of course, find more expensive oil with a better quality blend, so do your research if you’re going to use it in a vintage motorcycle and not your ride-on lawnmower to find out what will be best for your needs.