if hes worried about water and recommending to mix engine lube oil to prevent this - Please stay away from this person. water will get in regardless of you adding, mobil1, PSO, caltex oil or even tullo, dalda or planta to your fuel load - if water is already part of it
diesel and engine oil are miscible but still are less dense than water making it float. In any case if you have water in the fuel tank you need to keep an eye on the sediment tank warning and drain it immediately or flush the tank out.
If lubricity is a concern then you need to add some "ashless" 2 stroke oil to the tank at a max of 70:1 ratio - and not more, Dumb moronic mechanics in pk think that regular 30/40 engine oil is also 2 stroke oil because ricksha runs on it. Its not - 2 stroke ashless oil is literally blended to ignite unlike lube oil which is blended to prevent atomisation and ignition.
btw - the vespa 2 stroke ricksha/scooter engine runs very very good if you blend 2 stroke oil in the petrol and not lube oil. - But - you cannot fix ustaad/shaagird syndrome because apparently the manufacturer are idiots.