I would never switch from regular to synthetic. Ive heard stories and actually seen an incodent where it blew up my neighbors oil pump and put a hole through his oil pan.
This is a real tricky question but everyone I ask always says if you use regular oil then say with that.
From what I put together, when you use regular oil a black gunk build up occurs on all the gaskets that touch oil. In other words it creats even more of a seal. When you switch over to synthetic, it cleans all that gunk off off the seals and starts to deteorate the seals untill you get an oil leak. IE: valve cover.
In my neighbors case, it cleaned all the gunk off the seals, cluged up the pick up screen in the oil pump and it caused such back pressure the pump exploded and blew out the oil pan.
I change oil on these cars daily and BMW reccomends any vehicle 1999 or below gets castrol 15w40. If you buy it from your dealer it should have a green lable on the front. The synthetic has red. For both my cars I put 15w40 in year round with no problems.
But like I said, eveywhere I go, parts stores, friends, co workers they all say the same thing. If your running regular oil, dont change to synthetic. But nobody knows why.