If you are asking about synthetic oil affecting your warranty......NO as long as you use GM's recommended weight of oil and the oil meets GM's certification requirements. You will find the requirements for both of these in your vehicle Owners Manual.

 

I picked up about the same in MPG. I get as high as 33 at a steady 70mph for the 35 miles I drive to work at 6:AM. With a mix of rush hour traffic in L.A. in the evenings and little to no traffic at 6AM going to work I get 29mpg average by weeks end since I switched to Mobile 1. My dealer did the change for me at 3500 miles.

 

I've got a case of Q-Horsepower full synthetic I got last year which I'm going to draw from for next oil change. I'm hoping to see an improvement in mpg but won't necessarily be disappointed if none is apparent. I got the Q not due to their silly horspower claims, but due to the fact it was free after rebates.

I intend to use another PF457G Delco brand filter for the oil change, same as what is installed now, so the only variable will be the oil type changing from conventional (Mobil Clean 5000 5W30) to synthetic (Q-Horsepower 5W30). I'll post any mpg changes. Probably be doing the oil change in about 3 or 4 more weeks.

 

December 2007 GM TechLink article by GM Powertrain Fuels and Lubricants Group about long standing "Oil Myths":

http://216.182.211.32/techlink/image...7e.html#story1

 

GM6094M Approved Motor Oils list revised 02-28-2008:

http://216.182.211.32/techlink/image...ts_02_2008.pdf


(right click, "save target as" comes in handy for this one...)

 

I changed my oil and used Full synthetic this time my MPGs went from 22 city to 25 city, highway went from about 30 to 32mpg. When I read this thread before the oil change I thought "theres no way that this would increase my mpg" well I was proven wrong.

 

Synthetic oil improved my MPG.
I changed my oil at 1000 miles and was getting 26 mpg average.
After putting in Castrol Syntec I'm getting 28 mpg average.

 

I bought a used 06 HHR (16,00miles) last December and use conventional oil. Thinking about changing to synthetic but not for gas mileage. The worst gas mileage I have gotten over the 5,000 miles I've put on the vehicle is 30.5 and I have gotten as much as 34mpg. I don't consider myself to be an easy driver...am I just lucky?
My question on synthetic is whether there's a down side regarding the engine operation. I'm considering changing to synthethic on my next oil change just for the convenience of not having to go for oil changes every 3K.
Any input?

 

The only negative effect you will see is in your wallet.
Synthetic is better for the engine.
And as far as changing the oil every 3k, it is not necessary. Use the GM recommended intervals from the oil life indicator. I'm sure others will chime in....

 

I agree but i don not ever go over 7000 miles on the same oil regardless of what the DI says.