I can understand why some type of heavy machinery would probably require different oil to a lawn mower but there are lawn mowers that require different oil to other lawn mowers.
If the answer is “because that’s how they were designed” then why? Wouldn’t a common oil make more sense?
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Different engines work at different RPM and different work loads to do their jobs correctly. A push lawnmower uses direct drive and has to push at a certain RPM to achieve the desired results. A riding lawnmower needs to drive the lawnmower and also have a PTO to drive the blades. With the PTO, you can use gear ratios to drive the blades at the desired speed. One operates at 7000 RPM. One is at 3500 RPM. One needs a thinner oil, one needs a thicker oil based on their expected load and drive style.
A common oil would make sense from your perspective, but you’ve gotta remember that you’re dealing with engineers here.
Because different operating temperature, parts clearance, contact pressure and operating speeds will require different kinds of oil to work optimally.
You can force all engines to use thicker oils, the engines will work and often better protected from wearing out. But thicker oils usually means harder to start in cold weather and increased viscosity in normal operation, which drags down the engine so it consumes more fuel and produces less power.
Using thinner oil gives you better efficiency, but you risk going right through the oil film during high bearing load and high temperature operating conditions.
Modern engines are designed with tight clearances and hopefully low viscosity drag to run efficiently, with Japanese engines going much more aggressively in this direction. Usually they recommend 0W-20 oil and sometimes as low as 0W-8. Americans don’t tend to care about mileage apparently so oil choices for them are usually thicker.
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