2004 JGC Oil Viscosity

I just purchased a 2004 Grand Cherokee (4.0l I-6). It was a program
car and had 11,000 miles on it when I got it. Since it was a "program
car" and I had no idea what kind of maintenance had been performed on
it, the first thing I did when I got it home was change the oil. Now,
for my first mistake: I very quickly thumbed through the owner's
manual (that's all I got, by the way, was the owner's manual, the
maintenance log was long gone) and saw 5w30 as the recommended oil
viscosity. So, this is what I used - Mobil 1 (synthetic) 5w30 and a
Mobil 1 filter. After the oil change I reread the page and found out
that that particular recommendation on viscosity was for the 4.7l V-8
engine. The specs for the 4.0l were on the previous page. I now know
that the manufacturer only recommends 5w30 in the 4.0l I-6 in very cold
weather and 10w30 for most conditions. I live in Southeast Kentucky
and oddly enough we have had somewhat of a "warm spell" the last couple
of weeks. Temps have been in the upper 60s on some days. So, my point
is this: Since the weather has been quite warm and since I'm using a
viscosity of oil that is not recommended for the temperature range(even
though it's synthetic) am I hurting my engine? I don't want to think
it will be a problem, but I'm open to opinions. I will definitely
switch back to 10w30 on my next oil change, but in the meantime I'm
hoping not to cause any additional wear. Thanks in advance for your
comments!

Hi Neil,

Both oils have the viscosity of a 30 weight oil when hot. The 5W-30 is
simply more viscous when cold. You won't hurt the engine using 5W-30 for
'cold' starts in warm weather. In fact, for a new engine like yours with
presumably good seals, no need for the thicker 10W-30 to prevent leaks.
You're better off with a 5W-30 or even 0W-30 for faster penetration on
start no matter what the weather.

Steve
http://xjeep.dyndns.org


Neil wrote:
> I just purchased a 2004 Grand Cherokee (4.0l I-6). It was a program
> car and had 11,000 miles on it when I got it. Since it was a "program
> car" and I had no idea what kind of maintenance had been performed on
> it, the first thing I did when I got it home was change the oil. Now,
> for my first mistake: I very quickly thumbed through the owner's
> manual (that's all I got, by the way, was the owner's manual, the
> maintenance log was long gone) and saw 5w30 as the recommended oil
> viscosity. So, this is what I used - Mobil 1 (synthetic) 5w30 and a
> Mobil 1 filter. After the oil change I reread the page and found out
> that that particular recommendation on viscosity was for the 4.7l V-8
> engine. The specs for the 4.0l were on the previous page. I now know
> that the manufacturer only recommends 5w30 in the 4.0l I-6 in very cold
> weather and 10w30 for most conditions. I live in Southeast Kentucky
> and oddly enough we have had somewhat of a "warm spell" the last couple
> of weeks. Temps have been in the upper 60s on some days. So, my point
> is this: Since the weather has been quite warm and since I'm using a
> viscosity of oil that is not recommended for the temperature range(even
> though it's synthetic) am I hurting my engine? I don't want to think
> it will be a problem, but I'm open to opinions. I will definitely
> switch back to 10w30 on my next oil change, but in the meantime I'm
> hoping not to cause any additional wear. Thanks in advance for your
> comments!
>