First lets lay down some premises on why you would like to do this procedure.

1. You pamper and love to tinker around with your Vespa which is why
you are willing to go to this trouble.
2. You agree that the efficiency of your 2T oil is affected when you mix
different types and brands of oil. Mineral, semi-syn, syn (ester or
viscosity) and castor have different formulations that can affect one or
the other when mixed.

So, first things first, try to use up as much of your remaining oil. A post in Modern Vespa or was it Stella Speed has mentioned that even if the 2T sight glass is empty, you still have some oil left in your oil tank to get you to some distance but how far actually I can’t find any info. For this particular procedure, I did around 50Kms on an empty sight glass and was able to drain approximately around 160ml of 2T oil. How far that much of oil would have gone I really don’t know, please feel free to comment. The best way to do it probably is to take apart the tank to fully drain it, fill it up with a liter of oil, jot down the present mileage. When the site glass becomes empty, take apart the tank, measure what remaining oil is left, note down how much was traveled then do math, we should be able to get an estimate on how much oil is used per km and how many kms can be traveled when the site glass reaches empty. This is all of course just for information discourse.

What you will need-latex gloves, a 50ml disposable syringe, a container for the oil you plan to drain, necessary tool to remove the hose clamp of your 2T oil, a rag (things can get slippery and messy) and your ever reliable Haynes Manual.

1. Remove the oil cap which is under your saddle.
2. Remove the engine side cowl.
3. Identify the carburator casing, there should be a hose, clamped
and connected to it. In my PX200E it is a translucent hose with a wire
retaining clamp. Before actually doing this procedure it is best to
ask your friendly Vespa wrench to point out this hose to you. You
can also refer to your Haynes manual.
4. When you have removed the retaining clamp, pull the hose from the
inlet, quickly cover the end of the hose with your index finger and let
the oil drain into a container by gravity.
5. With the 50cc syringe, I am able to do things much faster. At a certain
point dripping will stop or you cant aspirate anything with the
syringe, this is when you have to get your Vespa off the center stand
and ask someone to lean your Vespa a tad towards the engine side.
Remember that fluid measurements of your Vespa including gear oil
and brake fluid levels should be measured with your Vespa level on the
ground and not on the stand. Some owners blow air into the oil filler
opening, others just let gravity drain it over night.
6. When nothing left can be aspirated by the syringe. Return the hose
back to its fitting, return the wire retaining clamp. Clean up, return the
cowl.
7. Fill up with a liter of your new oil. Others would rather fill up till the
sight glass is full only. Then wait till the sight glass is empty then fill
it up with the remaining oil. Argument for this is that the old oil is not
completely drained and constant dilution will wash out the old oil.
8. Start your PX, let it warm up then blip the throttle a few times when
warmed up.

When I change to a different oil I also clean the spark plug with a wire brush and check the gap.

Again I am not an authority. Someone will probably question my credentials as to being a Vespa wrench. However this is what I have been doing for the past 5 years when I change to a different type or brand of oil and it apparently hasn’t given me any problems. Most Vespa shops and wrenches here in the country do the same thing when the service asked from them is to drain the 2T oil unless the owner specifically wants the oil tank to be actually removed which is really a tedious job. For this particular change, I am going from “Repsol Syntetico a synthetic technology oil” to “Repsol Competicion a fully synthetic performance oil”.

The oil you drained is clean and can be used for general home lubrication, hinges, bicycle chains, tools, etc… Otherwise dispose of it properly.