1) NEVER ever let Paypal convert it to SGD for you, let Paypal pay the transaction in the native currency of your transaction.
2) Irregardless of the way Paypal pays (either in SGD or native currency), the amount you pay in your SGD credit card will always be in SGD (unless yours is those few dual currency credit cards)
3) So why the hassle of doing Point (1) when Point (2), everything will eventually be in SGD anyway? Simply cos Paypal's conversion rate is inferior to Mastercard/Visa + bank's conversion rate. You will pay less by letting Paypal charge in native currency and letting the bank do the conversion.