Oil clarifications

Aloha gang =). Please be gentle on this honest mistake. I have A 2011 GMC Acadia that recommends full synthetic oil. I have been taking it to a local firestone and the first change I had told them it required full, they said they know. So sending the Mrs. to have future oil changes since they “know” because I was out of town. 3 changes later i find out they have been using synthetic blend and it was changed every 4000-7000 miles. Now it is in the shop, the 3rd piston seal broke and there is loss of compression could this scenario be the cause? Please any info would be appreciated.

By “piston seal” do you mean a ring? Did you break a piston ring?
By “the third piston seal broke” do you mean that this is the third time this has happened or that the lower oil ring broke?
How many miles are on the vehicle?
Have you been monitoring your oil level? Or do you let it get low?
Did the shop when they did the diagnosis mention anything about sludge?
Has the vehicle had other serious failures such as overheating?

Lastly, are you the original owner?

As a non-professional, are you sure GM speced only synthetic, or oil that meets their specs? My Acura called for just 0-20 and the dealer was using Honda blend instead of full synthetic, so I kinda question that a blend in the right weight and quality wouldn’t have been OK. Is this a warranty item and are they claiming improper oil? I only use Mobil 1 and change at 5000, but still I don’t think it should have caused a problem. But I’d want to know specifically what Firestone was putting in there.

The owners manual for your vehicle specifies the warranty requirement to use a motor oil that meets GM’s Dexos specification. But oddly, then they say if Dexos oil isn’t available, you can use a 5w30 oil that has the API Starburst symbol.

Many synthetic blend oils do meet the Dexos specification.

The owners manual is at:

I can’t comment on if your engine problem is related to the use of a synthetic blend. I’m inclined to think not, but there’s not enough info to know.

You should still be under the power train warranty. It’s usually 7yr 70k miles.

if a warranty was involved you would not be asking about oil grades. unless the dealer is denying the warranty work due to lack of paperwork on oil changes or paper work that does not specify the proper oil grade.

Dexos spec oil is semi-synthetic, but I wouldn’t trust a a chain to keep track of vehicle specific oil types. No offense, but was the oil level checked and maintained between these 4 to 7k oil changes?


If oil that did not meet GM’s specs was used, I would think the power train warranty is in jeopardy

Mobil1 0W-20 full synthetic is Dexos approved.

The Firestone Complete Auto Care Centers in my area use Kendall oils. I thought that was a nation-wide corporate decision, but could be wrong. Kendall GT 5W30 semisyn and Kendall GT 0W20 are both Dexos1 approved oils. If the issue of approved oil use comes up, contact the Firestone center and confirm that the oil they installed was Dexos approved. The information might well be on the receipts for the oil changes done there.

I think OP is already aware it is best to use only an oil recommended in the car’s owner’s manual. But I’m of the opinion that as long as a good quality oil of the proper weight was used, maintained at the proper level, and changed (including filter) on a reasonable frequency, then it is unlikely that the oil would cause a piston ring to fracture. Using the wrong oil is more likely to cause problems in the upper part of the engine, the camshaft, variable valve timing mechanism, and if your car sports one, the timing chain mechanism.

It may be that you just got unlucky and had a defective piston ring which eventually failed is all.

I’m still wondering of a busted piston ring is even what the OP was tying to describe…
I hope the OP comes back. It’s tough doing diagnosis over the internet via a one-way conversation.

I suppose it is possible the OP is referring to a valve stem seal on the number 3 cylinder. Hopefully the OP will clarify …

I hope so. I really would like to help.

There’s a lot of missing info so some blanks need to be filled in.
Seal means ring?
Which shop?
Engine already torn into to determine this?
Vehicle towed in?

If the compression is extremely low or zero there has to be something very serious going on if this is due to a ring problem. One also has to wonder about oil level at the time and any warning signs before the bad news hit.