Is it correct to use "but" as a positive connecting word?

I'm wondering if it is correct to use but as a positive connecting word. Below is a small example:

... is the perfect opportunity to improve my knowledge but also gain new experience with ...

Is this allowed or should I rather use and?

But always contradicts, and is therefore almost always negative. You can use it as "positive connecting word", but only if it's contradicting a previous negatively-oriented statement:

... not only is this the perfect opportunity to improve my knowledge but also to gain new experience with ...

If it's not contradicting anything, don't use but. Use and.

But is synonymous with and. I.e, if

• Bill went early but he got back late.

is true, then so is

• Bill went early and he got back late.

Ditto for when it's false. This is the definition of synonymy. Synonymy is a semantic relation; the difference between but and and is not semantic, but rather pragmatic.

One uses but instead of and when one wishes to express personal surprise at the conjunct following but, for whatever reason. Normally this is a matter of some expectation of the speaker's being violated, and the speaker may believe that the addressee has the same expectation and will be similarly surprised. This belief, of course, can be in error, like any belief.

But, however, is not negative per se; it does not trigger NPIs. I.e, this is ungrammatical:

• *Bill got to the auction late but he bought anything.