newer motors seals can handle synthetics better than old school motors that have original seals. the b20 should be able to handle synthetic w/ no prob. the question you have to ask yourself is, is it worth the extra for synthetic? if this is a fresh rebuild and you are going to be changing the oil at 100mi, 500mi, 1000mi, etc during the break in...do you want to spend $4-5/qt. if the motor is already broken in then the benefits of syntetic are there in terms of thermal breakdown and longer oil changes if you are not into changing your oil every 3k miles. unless your motor is got tons of miles on it, it shouldn't be a problem to make the switch. if you are boosted, synthetic is even better because it doesn't break down, burn, and coke inside the turbo as easily.

weigh your options as well as your wallet.

i stand behind the use of full synthetic and highly recommend it...it just depends on your situation.