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Hey guys, I have a 2005 G6 with 152,000 miles. I've been using Valvoline high milage synthetic blend 10w-40 oil. I was wondering if I should keep using that or switch to a synthetic blend or a different weight? I live in NEPA so the temperature has a pretty drastic change from winter to summer and I know the oil weight is based on temperature but I don't know much about it. I have also noticed a slight knock when I first start my car cold but goes away when it warms up. Any help or insight into what oil I should use or anyone else uses with similar conditions would be very helpful and appreciated! Thank You!!
 

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how about what the manufactuer recommends which is 5w-30
 

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That knock is probably because (when cold) the oil is too thick to get around fast enough. I'd stick with the manufacturer recommendation of 5w-30.
You could probably run 10w-30 during the summer though.
 

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You can go up or down in viscosity... but its generally really frowned upon for weights..

For winter you could go 5w-20 or for super hot climates 5w-40.. this is all about viscosity..

Oil weights is the size of the oil molecules.. 0w would be too fine and could be burned by the engine... 10w would be too large and not get in the cylinder walls and could cause excessive wear.. generally the tighter the engine tolerances the finer the oil weights..


so.. here is the deal.. run 5w-30 unless you have really good reason.. 5w-20 if you see temps BELOW -20* and 5w-40 in the summer if you see temps ABOVE 110*.. other than that.. use the recommended 5w-30 and nothing else..
 

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Stick with the synthetic blend and stop using 10w-40. Like CruisinG6 said you get that knock in cold winter because of the oil weight. Through time you can start damaging your internals even if it's for a short period during start up. I stick with 5w-30 but when winter arrives I switch to 5w-20 if I get the chance, and I've always used Mobil1.
 

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I use 5W30 full synthetic in my GT - one oil change a year. I add about 1/2 a quart after winter and then change it every July.

I don't like synthetic blends and/or winter oil. The manufacturer makes more money off of this oil than any other, because they can add as much, or in most cases - as little full synthetic stock as they want. Some blended oils have as little as 10% full synthetic. I would go with full synthetic 5W30, or conventional 5W30. As I change my oil only once a year and I have very little burn off 5W30 is the best choice. Temps can go -25C in winter and easily reach 30C+ in the summer.

10W40 will also reduce mileage. 5W30 reduces friction, and a full synthetic 5W30 will reduce it more. This is one of the many reasons why you can drive longer using full synthetic oil. Full synthetic also flows to vital engine components (top end of your engine) quicker in cold weather.
 
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