10w40 is NOT going to hurt a thing and cap says 10w30. If you read the caddy forums when the miles get high in these engines you'll hear a slight thud at idle. That's the main thrust bearing and to silent it they recommended a thicker grade. Some even put 20w50 in these. That's to thick for even me. Car runs fine in it. Probably shear down to a 30 anyway. Engine is a 4.5L with MPFI. She's strong to. If I not mistaken these engine where weird and aluminum block with iron heads. Its similar to the HT4100 engine.
I didnt say ot would hurt it, it makes it sluggish and throws away bottom Torque.
Big issue with these is coolant wearing through the back of the block behind the trans bellhousing.
Sometimes its best to just drop the subframe to work on it.