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!