I don't agree with sealing the suction hose in the dipstick tube. You're not trying to create a vacuum in the crank case. I remove the oil filler cap to ensure that vacuum isn't created in the crank case as that would only make it more difficult to extract oil.

Counterpoints?

When I got my Topsider, the nipple that the suction hose connected to was loose allowing air to leak into the chamber. The Topsider people sent me a new fitting and it's been fine since.

See if it'll hold pressure. Not as telling as submerging it but a whole easier.

Sixto
91 300SE
87 300SDL
83 300SD