Castrol "XL" ( listed in the welbike " operation manual ") Is in fact straight 30 oil, low detergent.
They do also list Castrol "D" in the same manual for the gear box , unless it makes the clutch drag, then it is recommended that the " XL" oil is put in it.
If you go here : https://classicoilsshop.co.uk/castrol-classic-xl30 , Castrol does in fact still manufacture the "XL" and the "D" , and it is sold in a few different places in the UK.