I would definitely clean it up and seal it again. Use a matte sealer.

Are you sure you caused the oil stains? I'm wondering if the adhesive they used is bleeding thru?

I think I've caused the stains, they are small and it looks like a drip. I'm considering re-sealing it. But now my question is: do I need to strip off the old (new) coat of sealer? Based on some other posts, I looked into Miracle 511 Porous Plus Penetrating sealer, but I'm not sure I can use it for counter top. It looks more like for flooring.
Any suggestions?
thank you

I would have the manufacturer and installer work this out. There must be some type of guarantee/warrantee included.

Good advice from RRM1 but in the meantime I'd try to remove the stain before applying any sealant. The link below has some ideas for stain removal, but I'm not sure if they all apply to the type granite you have.

Here is a link that might be useful: granite stains

The manufacture won't be able to help you here as that manufacturer is Mother Nature. The fabricator won't be much help either, as the stone is performing as is it's nature to perform.

It's a well known issue that honed black counters show every bit of oil based touching and hold on to the oils in their texture. I'm surprised that you didn't come across this information in your research for your counters. Sealing doesn't really help, as the culprit isn't the porosity of the granite itself, as black granites are almost completely non porous. The issue is the honed texture that traps the oils. It's the difference in a wood surface sanded with 80 grit sandpaper and 150 sandpaper. One has more surface area to grab you because of the coarser finishing. Black granites are dense enough that sealer won't penetrate and can actually exacerbate the "stain" issue.

Some people have had good results from removing the sealer and then using Method granite cleaner, as that puts down a silicone oil coating that minimizes the appearance of oil stains. It is supposed to be safe for food contact surfaces, but I wouldn't make a sandwich on a counter treated with it.

I have brushed jet mist countertops and had the same problem with oil stains when they were first installed. I removed the stains using a poultice of acetone and baking soda. (try a google search for 'granite poultice oil stain' for directions on how to do this, there are other things that can be used besides acetone and baking soda, but those worked for me). After the stains were removed I gave the granite two more coats of sealer and haven't had any stains since, in over a year. I think the fabricator didn't account for the fact that brushed or honed granite is more porous and absorbs more sealant than polished granite, so the single coat of sealant they applied was not enough. Hope this is helpful -- I remember how upset I was with those oil stains but now that it's properly sealed I love my jet mist!