Incorrect Oil Weight
 

My father put 10w-30 Mobil 1 Synthetic instead of the approved 10w-40 in his 01' c2. Could this cause any harm to his engine?

I would not run 10W in cold weather or 30 in really hot weather. But neither is likely to cause any real harm in moderate temps. If you have a 02+ with VarioCamPlus you might get some problems with the lifters with the 10W.

By the way I think the approved oil is actually 0W-40, but I never run 0W-40. 5W-40 in the winter and 5W-50 in the summer for me, and I use Castrol Syntech as it has more ZDDP.

whats with the oil threads lately..... remember the days of disregard threads.

Cars are coming out from winter storage maybe?

If it were me, I'd dump the 10-30 and replace it. The 30 is a bit thin for warmer driving--and the Sacramento area is definitely "warmer" during the summer (I used to live in Davis, right next door). Plus the Mobil 1 seems to "thin out" as it ages, and the 30 will become a 25 or ??? Whilst the 30 might be okay, I personally wouldn't take the risk with a $20K engine using too thin an oil.

Assuming the oil was just changed, you needn't replace the filter. Just drain the 10-30, replace with 5-40 (I'm with Dharn55 on not using 0-40), and you're good to go. Easy DIY job.

Sorry I was in a hurry when I posted the thread! Thanks for the input!

5W-50 in the summer... Castrol.... Hmmm...

Never run anything but Xw-40...Have him change the oil. 0w, 5w or 10w but always use the -40.