Can i use engine oil 10w30 as transmission oil in my automatic scooter.I know the viscosity of both oils are different but the mechanics here are doing so .What would be the long term impact of it
Automatic gearbox oil has different characteristics compared to engine oil you should use the oil specified by the scooter manufacturer, or a rated equivalent.
Using the incorrect oil will probably impact on the performance of the auto box and its life.
It may move but it will not operate properly. As stated : The additive packages are different. A friend once put "gear oil" in a motorcycle standard transmission ; it wouldn't work until he took it apart cleaned it and put in new clutches. In the 50's Chrysler (Plymouth) made the stupid move of engine and auto trans sharing the same oil . It sort-of worked ; however the transmission was designed for motor oil. Sludge from the engine messed-up the transmission.
The choice of oils for engine vs transmission is largely a function of the additive package. Generally transmission oils have better extreme pressure characteristics, which are better suited for meshing gears. They are usually also thicker, although that is not drastically the case. Gear oil viscosity numbers are similar to aviation oil numbers, which are generally 2x the normal engine oil ratings.
40 weight engine oil is about 80 weight aviation engine oil is about 80 weight gear oil.
My take is that in a pinch you could use a 10W-30 or 30W oil, but that it would not be good for the conditions in your gear box or transmission.
It is worth noting that a scooter transmission probably has different needs than a transmission on a truck or tractor. But if your manufacturer specs gear oil, they have likely done so for a reason.
More critically, since the scooter transmission is an automatic, the additive package is even more important than in a manual gearbox. So for an automatic, I would simply avoid any non-rated oil because of the adverse wear characteristics.
The long term impact would be increased and abnormal wear. The short term impact would likely be reduced cost, until something started failing.