Synthesist level dip

[Kaldre]

"The synthesist uses the eidolon’s base attack bonus"

If I take one level dip into synthesist and rest of the levels in my other class am I forever at 1 BAB?

or would I be using the eidolon's BAB for that one level then add my BAB from the other class?

[Redneckdevil]

When u are fused, yes u are stuck with the base attack of it because its mimicing ur movements so it is making the attacks not ur character.
when u are not fused, u use ur own attack bonuses because the eidolon is no longer the one attacking but ur character.

Moral is if ur focusing solely on it to do physical attacks, dont multiclass to heavy and keep most classes as ur summoner. If ur focusing on spellcasting offensively and/or open up ur spell options then multiclassing is a smart move.

[Azten]

There's an FAQ that addresses this.

EDIT: Here it is.

[Kaldre]

Thank you that makes more sense.

When fused, use the eidolon's BAB instead of the summoner's class BAB, and add in BAB from other sources as normal. For example, a fighter 19/summoner 1 normally has a total BAB of +19 (+19 from fighter, +0 from summoner), and when fused with his eidolon this increases to +20 (+19 from fighter, +1 from the 1st-level eidolon).

So they would get the bonus from the other class