What in the world are you thinking? The instructions for preparation and dosage are there for a reason and should be followed exactly. The cardiac glycosides in oleander cross assay with digitalis/digoxin. Three tablespoons is well withing safety limits, but 10 could be needlessly flirting with disaster.
Never forget that you are making a remedy from a plant that is so toxic in raw form that eating as little as two leaves can be fatal. Either follow the directions exactly or do not use it.
There has has never been a fatality from taking oleander extract and thousands have used it - I would not want you to be the first! The most serious event I know of was from a woman who decided for some crazy reason to take half a bottle of the extract at once - and she had to be hospitalized overnight with a racing heart.
Don't do it! More is not always better. When it comes to things that have medicinal properties it can not only be dangerous, but even fatal.