yes he was right but it needs a bit further explaination.
both thick and thin have advantages and disadvantages..
extreme racers and mpg recorders even use kerosene (or diesel) instead of engine oil.. because the thinner the oil is the less the moving parts has to face resistance and it will floew eaily.. consider moving your finger through petrol vs engine oil.. u will see that you finger has little resistance when u move it through petrol.. same is the case with thin engine oil.. parts move faster. but the down side is that the lbrication is not suficent and parts gradulay detoririate and eventualyy friction increases to the extent to jam.,. this is the case with 'extremist mpg/economy gainers' and their bikes get destroyed after setting the 'record' ...
so in the above situation there was a hell of difference between viscosity .. but in reality the different engine oils have little difference in viscosity.. so in theory thiner oil gives more mpg/economy/ or km/ltr but in the long run not good for engine durability.. also if u mainly run at slow speeds then thinner oil is good as it quickly starts its actions and so it is recomended and it here protects engine. but at high speed it doesnt protects engine well...
thicker oil on the other hand is good at high speeds for engine protection but at least in theory it will reduce top speed and acceleration.. and at low speeds it will not be good for lubrication as it will take long time to get to its working state.. and parts will face friction.. and in case u turn on and off ur engine too frequently thick oil has the disadvantage of providing good lubrication.,,, as the fact is that bikes get most of damage at start,, as at start lubrication is not good.. so in this case thin oil is recomended...
but after noting that there is very little difference in engine oil thickness and riding style is not consistant any oil (thick/or thin) will be ok..
hope i m correct ..?