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?
In: 2
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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