You can use the oil, definitely it is a very good oil and superior to others available in market.
Engine oil cant have any measurable effect on ur topspeed of bike
i'd recommend you to not to use any oil more than 1000~1200 kms in a small 70cc engine and stick to 20w-50 grade in summers and winters both. In 70cc if you drive 50kms constant you will most definitely feel that bike is heated up and clutch behave in a different way, but surely LM keeps bike cooler and clutch happier than other oils.
Regarding price idk, check with local suppliers.