After or around 200k I've always switched my oil to 10w30. It gets real cold in Canada so 5w30 for winter. My Buick Century with 3.8L got over 250K with not one problem ran great, very quick.I got over 350K on my Cutlass with 2.8L MPFI never touched the block or heads it had a bit of blow-by but never smoked out the back.Currently my Lumina's 3.1L has just turned 253K and runs awesome. No blow-by, no oil burning, not even a leak.
Although in my opinion the 3.4L should have always had 10w30 and you should probably use synthetic; although switching can possibly create leaks, possibly!!! The Twin-Dual is a very complex motor a puts a lot of stress on all the systems of the car, it is not one that takes well to neglect. Routine maintenance is the key with these motors; I've seen many trashed by 100k very sad to see cause these engines rock.