10w30 even in cold winters?!

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Sep 7, 2002
Jefferson, Wisconsin
My girlfriend just picked up a 1999 Grand Prix GTP (VERY enjoyable car to drive BTW) and the oil cap calls for 10w30. After I run Auto-Rx through it I plan on running Mobil 1 10w30 in it. Should I still use the Mobil 1 10w30 even in the cold winters here in WI or should I switch it to 5w30 in winter? Aside from all that... would it be safe to run Delvac 1 5w40 in this car? (It would be 1 gallon/4 quarts of D1 and then 1 quart of whichever grade of Mobil 1 seems best to go with) Thanks! [ June 26, 2003, 02:46 PM: Message edited by: Intelman34 ]
I would switch to 5w30. I ran 10w30 last winter, and when it was down to -10F I was cringing. The engine still started, however I'm sure my increased iron content was partially due to the thicker oil.
A few years back I performed a crude viscosity test out in my garage. Temperature outside was about -20 F, about 0 F inside garage where I had stupidly stored a few cans of oil. (Now I always store oil in the winter in my warm basement.) Anyway, I tried pouring the following oils: dino 10W-30, dino straight weight 10W, and Delvac 1 5W-40. The dino 10W-30 poured like stiff molasses, the staight 10W was only a little better, but the Delvac 1 poured almost like summer. And my old (200,000+ mi) 70 Impala started every day inside or out with the Delvac 1.
I ran Amsoil ATM 10W-30 in my Grand Prix GT in our miserable winter last year. I had some scary wear numbers, so I would think that 5W-30 or even 0W-30 is called for. I'm currently running Amsoil's Series 2k 0W-30 to see how well it takes to the racing and hotter temps of the summer.
What does the owners manual for the 3.8 say? Do they reccomend 10W-30, even below 0 deg? I know the '99 3.4 says 10W-30 is OK down to 0, but 5W-30 is recommended.
This is probably a subjective statement: I used both Delvac 1 and Mobil 10W-30 in temperatures as low as -17F over a 9 year period here in SE Pa. Also used some in colder Williamsport, Pa. The oils were Mobil 1 and Amsoil. Can't verify the wear numbers.
It seems to me since Synthetic pours and flows easier that a 10/30 syn would perform similar to a 5/30 dino as far as flow is concerned. That being said I wouldnt see a problem running 10/30 syn year round.
I used M1 10W30 last year with the "brew", which jacked up viscosity a bit, and the engine started without a hitch. Wear numbers were only slightly elevated. So running undoctored M1 10W30 should not present a problem.
Mobil 1 10w-30 is is not much heavier then the 5w so I don't see what the difference would be. [ June 26, 2003, 05:19 PM: Message edited by: buster ]
It all depends on how cold it gets where you live, whether the car is garaged, and whether you use a block/oil heater during the winter! I run M1 15w50 year round in one car with 200+k miles and no problems. But, the newer cars get M1 5w30 in the winter and M1 10w30 for warm weather changes.
It in Jan/Feb it gets near zero and sometimes even below (without windchil obviously) on the cold days. The car will not be in a garage and no block heater is used. I am thinking Mobil 1 5w30 or Mobil Delvac 1 5w40 in winters.... [ June 27, 2003, 06:16 PM: Message edited by: Intelman34 ]
Patman while I see your point. My point would be what is the marginal improvment in cold start between M1 5W30 and M1 10W30. I only start my car twice a day. Once the oil temp comes up I still have to worry about shear(I own a shear crazy toyota). I also want to have some hydrodynamic film strength. See I am worried about what happens once I am on the road. I occ hit 6500 rpms. I know that M1's new SS 5W30 is alot better then it was but is the shear resistance there??? I am thinking that if the AW add's are plated up on the parts then what is an extra tenth of a second?
My wife drives a 98 GP GT that we bought new. I've always ran 10W-30 in it, even in the winter. I've run Mobil 1 10W-30 in it for the last 50,000 miles with no problems. Car has 92,000 miles on it now and still runs like new. I would stick with the 10W-30 and probably use a synthetic since the car has a supercharger. Wayne
I used 10w-30 all last year (winter even) in my SHO, and didn't notice any difference in the time it took for my oil light to go off, or for my engine to sound "lubed", between the summer & winter months. I bet my cams took a bit of a beating though, as I'm sure it took somewhat of a longer time getting to the heads. Then again maybe not [Smile]
Even if the 10w30 will seem to work out ok, you can guarantee yourself that a 5w30 from the same brand will flow quicker on a cold start. In other words, if you're using 10w30 Mobil 1 right now, switch to 5w30 Mobil 1 and you will get faster flow. Obviously if you compare a 10w30 synthetic to a 5w30 dino, it might be different, but my point is, why not go for the best flow you can get? Isn't the lowest engine wear our top priority here?
What about a good quality Dino... Pennzoil 10W-30 for example? Should I switch to 5W-30 in the winter months? I am in PA, and there are only a few days a year where we see temps approaching < 10 deg F.
novadude, I am going to take alot of heat on this one but here it goes. I would not run a dino 5W30 in anything!!!! I especialy would not run 5W30 in anything that that the manufacture recomends 10W30 in! So seldom does GM recomend 10W30 with C.A.F.E.! They must have found that the 5W30 dino oil was not giveing good enough protection because all L67 engines recomend 10W30 while the L36 recomend 5W30! THe L67 is SC 3.8L the L36 and L27 are NA 3.8L. Prior to increaseing Cafe pressure all 3.8V6 recomended 10W30. THe oil pressure spec. in the service manual explictly state that all oil pressure reading for warranty work are given @ operateing temp with 10W30 oil.
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