Indicative or subjunctive after "per quanto"

Question

Should the verb after "per quanto" be in the indicative mood or in the subjunctive mood, when it means "as long/far as"? Some examples:

• Per quanto ne so/sappia, lui è americano.
• Ti renderò felice per quanto potrò/possa farlo.

Answer 1

NOTE: my previous answer was completely wrong. It's been rewritten completely. Many thanks to DaG and egreg for helping me working this out.

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Per quanto is a conjunction that can introduce two kind of clauses. The choice of mood depends on the type of clause.

• Proposizioni limitative. These are subordinate clauses that specify the ambit of validity of the main clause. They can be both at the indicative and at the subjunctive, depending both on the level of formality and on the certainty that one wants to imply (and of course where appropriate they can be at the conditional). They are usually introduced by per quanto, per quel che or che. When introduced by Che they are necessarily at the subjunctive.

All the examples you gave in the questions are limitative, so all following forms are correct.

Per quanto ne so, lui è americano.

Per quanto ne sappia, lui è americano. (more formal, a bit stilted)

Che io sappia, lui è americano.

Ti renderò felice per quanto posso farlo.

• Proposizioni concessive. These are subordinate clauses that express a condition that ought to make the main clause false, but it doesn't. They are always at the subjunctive except when introduced by anche se, when they are usually at the indicative.

Per quanto questo problema sia difficile, ho fiducia di riuscire a risolverlo.

Benché abbai, questo cane è capace anche di mordere!