Hi I've just purchased a really clean example of a montesa 248 but have very limited mechanical knowledge and was hoping for some advice... I know it's a 2 stroke pre mix and been advised to use a 40-1 mix ? But I went to buy some gear oil and was sold engine oil (10/30 I think) is the gear oil & engine oil the same thing ? Do I just undo the drain plug and replace the oil ? And if so how much . Thanks in advance...