Regular 5w-30 put in vs Mobile 1
 

I have 98 E320 that has had Mobile 1 0w-40 synthetic oil in it up to 147k miles. i usually got 12-14k between changes on Schedule notification.

I had it in a shop that put in Valvoline 5w-30 regular oil. Is there any issues with switching to regular oil except the fact that i will need to change at 3-5k?

Oil changes
 

If the oil filter was changed also, and I assume it was, there should be no issues. I have change from dino oil to syn and back to dino only to discover I should have stayed at syn.
But that's my preferance. Using a flashlight and shining it down the oil fill hole, syn seemed to keep the parts much cleaner.
Because my car now has 247000 on it I do use the heavier 20/50 syn grade Mobil 1 Extended.

Tons of torque and yet to be rebuilt.

Conventional wisdom is that your car requires synthetic and MB requires synthetic - http://www.whnet.com/4x4/oil.html but that's only in the U.S. and only because of the FSS lawsuit a few years ago. Conventional oil is still allowed in other markets. You can be a guinea pig.

You'll need to disregard FSS and change it out at an appropriate (3kmax) interval; then go back to Syn, and NEVER to the shop who didnt know better.

Mark, I recommend changing the oil as soon as possible to Mobil 1 0W40. It's not worth the risk to run the Valvoline 5W30 for several thousand miles. As mentioned an experienced shop should realize the FSS engines require synthetic oil. That shop is doing their customers a disservice.