All,
I suspect this post may generate as many different answers as those that
reply ... :-)

I had an '03 Dodge Grand Caravan w/ the 3.3L. The manual called for two
oils: 10W-30 and 5W-30. The 10W-30 was for use at temperatures above 0 F,
whereas the 5W-30 (which is recommended) is for use at temperatures below
100 F; the overlap being between 0F and 100F.

Being in MI, I could run either all year long, but I chose to use Mobil1
5W-30 synthetic oil, hoping to get better economy and cold-weather starting.
At operating temperature, the van would develop minor tappet noise, and the
dealership felt it was best to replace the lifters and change the oil (to
regular 10W-30). When the oil change interval came due, I reverted back to
synthetic 5W-30 ... and the tappet noise came back also.

Now, I have an '05 w/ the 3.8L, which has the same oil recommendations. I'm
planning to use 5W-30 at the next change interval, but let's assume the
engine develops some tappet noise at operating temperatures:

With synthetic grades, has anyone ever mixed grades ... say 1:1 10W-30 and
5W-30? Any reason this should be a concern? I cannot understand why an
engine (designed to run with either viscosity) would object. I also would
not anticipate an adverse interaction between the oils. I would think that
the resulting viscosity would be ideal.

Thanks,
Chris