Oil is a pretty complex thing these days, and I don't pretend to know a great deal about it..... so, partly based on what I know, and partly based on some research :
The viscosity of the oil is just one of it's ratings. It also has pressure ratings and API figures.
The oil in an engine does more than simply cut down friction and wear by lubricating the pistons, bearings and other moving parts. It also helps to seal hot high-pressure gases, take heat away from hot areas and disperses it to the air in the sump, reduces corrosion and absorbs some of the harmful waste products of combustion.
As the loads in Diesel Engine will be much higher, they should also have adequate antiwear and EP properties. But one of the most important properties which is not critical for petrol engines oils is its alkalinity reserve to combat ill-effects of sulphur in the diesel fuel.
Sulphur present in the diesel fuel burns to form oxides of sulphur, which in the presence of water, form sulphur acids, which are highly corrosive to engine parts. Hence the need for high alkalinity reserve in diesel engine oils.
Why not use it in a Petrol engine ? For a start.. cost... The diesel oil is more expensive.
