I have know the 3800, and it's predecessor for many years... they are outstanding motors, and will hurt many Honda drivers ( and others as well) feelings with gobs of power, and knocking down 30 MPG + on the interstate in a boat that may actually float in some of the Buick's. I have never seen a need for anything other than 10w-30 on these engines, I have used that, and frankly I just never seen a need to move away from the manufactures recommendation. Also running PP, or PU for 5k is a total waste, I would not change the OCI ( and I am a fan of extending the OCI) but in this motor conventional at 5k is the best combo I know of, and I would not extend my OCI even with a synthetic unless using a UOA as a trend to establish a trend... still this engine works so good with any conventional 10w-30 like Valvoline, PYB, Castrol, and one that I really like is Valvoline High Mileage... as many of these engines develop some oil leaks when they get some age on them. I wouldn't worry much about the oil brand etc like I said conventional works great in this motor, engine oil is not what this engine depends on so much to get a long life, it is as mentioned before the intake gaskets.. also I would be checking the coolant out, and it's a cheap investment to change the coolant.. they last two we have owned has had to have the gaskets replaced, and one needed the engine replaced because of GM's cost savings disgrace to ruin a outstanding engine !!