Pontiac Repair: grade of oil, fuel econmy, thickner

Question
QUESTION: I have a 2002 Pontiac Booneville. 6 cylinder, automatic. The owner's manual says not to use any grade of oil other than 5w 30 or 10w 30. It specifically says not to use 10w 40 or any such thicker oil.   I want to know why not. Are there problems with a thicker oil, such as a weak oil pump or small oil tubes ?

I ask because I just bought this Bonneville and I'm only getting 25 mpg. My last car, a 1988 Corsica had a worn engine and I used 20w 50 AND an oil thickener. Without the oil thickner and 20w 50 oil I got only 8 mpg.  WITH the use of 20W 50 and STP I was getting 22 mpg.

This Bonneville has a good used motor installed in it, and the previous owner says he wasn't told how many miles was on it. I am not happy with 25 mpg.  Should I be ?  It does run just great.

Anyway,  the question I am asking is why can't I use a thicker oil in this engine ? I do realize the thicker oil would be uncalled for if the compression is adequate, but I am just curious if there is any engineering reasons why I cannot go to 10w 40, or 20w 50 if I want to because of low compression?   
Thanks
Dwayne

ANSWER: Hi Dwayne:

Use only 5 w 30 oil in this engine unless it has a LOT of miles on it the n use the 10 w 30. The thinner oil cuase less drag on engine part thus in theory better fuel econmy but nothing you will notice. also the newer engines have very tight clearances and the thinner oil flows into the clerances easier. if it has a lot of miles use the thinker stuff because the engine is "looser" A bonneville should gte around what you said your getting 20-25 MPG in mixed loop driving. highway/city. Hope that helps you out. Good luck :)

I for got to mention this, If you live in a cold climate the heavy grade of oil will have a bigger effect on fuel econmy then in a warm climate and also the heavy oils are more apt to sludge up!



---------- FOLLOW-UP ----------

QUESTION: Todd......  Would you please define LOT of miles ?  Are you talking 100,000, 200,000 300,000 or what ? The odometer is showing 140,000,  but the motor was exchanged at 138,000,  so that reading is obviously not correct to this newer motor. Wish I knew the miles on the newer engine,  but.......

Also, I do live in a fairly cold climate during winter. -17 is not unusual some days.  Did you mean the effect of a cold climate is to give my car more mpg, or less mpg?  You just say the fuel economy would be affected,  but you didn't specify which way that would be.  I'm just an 'ol dummy,  and need to have everything spelled out.

One other question.......  I went ahead and had my oil changed.  I replaced the 10w 30 (just regular petroleum oil) with a decent grade of synthetic 5w 30 (Motorcraft) and noticed that the oil pressure gauge showed an increase of about 30 psi. in oil pressure.  Is that normal?  The oil gauge normally showed around 40 psi before the switch from normal oil to the synthetic....and now is reading around 70 psi.  I have an idea that will also improve the engine's compression,  but not sure that is a correct assumption. That does, however, sound like the engine's internal lubrication will be improved, and benefit the motor.

Thanks for your input/response.
Dwayne

Answer
Your reading way too much inot this. Over a 100,000 mile is "a lot" of miles. At that point a 10 30 oil would be in order. below that 5 30 is fine. being that you do not know the miles that were on the engine I would recomend just 5 30m oil.. Not much need to use a "blended" oil. most of the regular crude oil products today are far superior to that oil oil from just 10 years ago. most of the new oils have ase rating on them of sg and sl on the lables and that is additives to the oild to help them from breaking down and sludging up quickly. Also if you use syntetic oil on an old engine with a "lot" of miles the engine will more then liklye start burning it and leaking it more so then regular crude oil. The synthetic stuff is much thinner when hot even know it's still rated 10/30 or 5/30 the structure of the chemicals are "thinner" this it will burn and leak oil. the 10/30 oil will not have much of an effect on compression. and you will not notice the fule milage difference. if your getting poor gas mialge it's not because of the oil in the engine it's something else.