Modern Vespa

I have low milage (under 1k ) T5 LML that had to be pulled apart because there was oil seal leak on the clutch side.

I changed all the oil seals and put engine together and tested for air leak. I only did it with pressure.

I tested it with 11psi, pressure drops to 6psi in 8minutes.
Air leak is between engine block and cylinder. I used new original aluminum gasket.

Should I replace the gasket or....?

Ensure there is no excess gasket from the engine halves that could protrude and interfere with the base gasket of the cylinder. Then place a smear of gasket sealer on both sides of the Base gasket, torque the barrel to spec, let it set then pressure test again

That should sort it out

I removed excess gasket but I'll check again.

What kind of gasket sealer do you have in mind?

I favour a silicone based one.

Really depends on where in tbe world you are as to brand names.

Honda Bond - seems to be available worldwide

OK, thanks