Switch to synthetic - how long before I change the oil again?

Hi - I have a 2000 Windstar with 75k miles. I made the switch to synthetic today with Pennzoil Platinum. The place I went to said I should get it changed at the regular 3k interval, which I know is BS (I've always had it done normally at 5k anyhow).

But when should I have this changed again? How much longer (if at all) does synthetic oil last?

Thanks for any suggestions you might have.

I run mobil 1 tri synthetic oil but if they have a sale on other mobil products I will try them. I still change mine every 3000 miles Im running mobil 5000 now but still will change 3k.. Depends on what type of driving you do.. If you do a lot of hwy driving 5000 ok, my wife does alot of stop and go driving around the city so I opt to change 3k.Synthetic does not break down as quickly as mineral oil and has better lubercating properties. I get better gas milage running full synthetic. I get 25 mpg running mobil 5000 and 27 mpg running full synthetic mobil.Mobil 5000 from what i was told is not fully synthetic.. Well thats my take on it.. and from what I learned from mechanics.. Of course every mechanic opinion is different...

I change my mobil 1 oil at 7500 miles with filter changes at half way. I do 85-90% highway driving.

Normal driving I would change oil and filter every 5000 miles. Use a good oil filter, not Fram or Quaker State junk!!