Modern Vespa

Rescuing this Vespa S and gas tank is clean and no rust. However it has some kind of sticky old oil in it. Any suggestion to remove these sticky oil to clean the tank?

You could start by sloshing a cup of gas around to help break up the oil.
Drain it off and then put in a degreaser like Simple Green and hot water. Shake it around for a good 10 or 15 minutes and see what you have.

If you need abrasives my favorite is black rock. But you can used pennies or old washers, nuts etc. too.

Depending on how crusty it actually is, I have used "The Works" toilet bowl cleaner which is a fairly safe acid. slosh it around for 20-30 minutes and even very rusty tanks are clean.

I've also used muriatic acid, but it is much harder to handle safely and will wreck alu parts that are in it's way.

Either way you will have to pull the tank to clean it.

If its good except for the gooey residue, I'd try the cup of gas suggested and if that won't dissolve it, I'd move up tp lacquer thinner or acetone.

Yea. Wd40 didn't do it. Degreaser gave up on it. I'm trying gas next. The blackish brownish stuff is sticky and soft like jelly silicone.

^^^ This...

Yep. gas worked perfectly.