Hi Bo, its not a good idea to mix synthetic with mineral based oil. I've been told by a motorcycle shop that the lubrication technologies of each type is too different. I was also advised not to mix two different brands in the same tank. This advice was confirmed by a cylinder soft seize on my scooter when I did so. In other words, just stick to one brand. I think if you let the current mineral oil disappear below the eye glass level and then fill the oil tank right up to the top with the synthetic oil and shake your scooter around for a while, you should get away with it without bleeding the oil tank dry first. Best wishes, David
Hi Bo, Another simple option is to pop the oil feed tube off the engine and drain all of the mineral oil out through the tube. (might take a while so make a cup of tea...) Save the old 2T for the lawnmower/chainsaw or squeaky hinges.
Once the old oil is drained through there is no significant residue and you can reconnect (remember todo this!) and refill the oil tank with your new 2T.
Personally I like mineral oil in PX engines as the viscosity is consistent with the design of the gravity feed dimensions. Not sure if the synth oil has the same density. Synth oil is most likely intended for direct pump delivery (2 stroke) systems where the viscosity isn't a factor.