I had read some of your postings about using the 15w40 oil for the 4.9 engine, should I just use it and stop using the Mobile 1 super sync oil or fully sync 10w-30 oil? what about summer and winter times?

I noticed that there is some kind of knocking ( engine sounds like a big blinder) especially when the car warms up, but when I start it in the morning and drive it for over 10 minutes the slight knocking starts, which explains the need for a heavier oil...

I guess you're asking Bbob, but just my 2cents.....I'm not convinced heavier oil is the cure/problem related to your knocking issue.

As for synthetics, I think they are good products, but I also believe there's nothing wrong with using conventional oils. In fact I believe regularly replacing conventional oils is better than running synthetics for extended oil change intervals.

Since the car was designed and validated using conventional oils, I say stick with it. But now that you've laready been running synthetic I am not sure about switching back....I understand they don't mix well.

Any of the oils mentioned will protect the engine fine and are more than adequate.

The only way to know if a heavier oil will help the noise you perceive will be to try it....sooo.....try it, it will not hurt anything and, at worst, the noise will be exactly the same. If the noise is the main bearing thump often discussed the thicker oil would tend to help it some.

Conventional oil and "synthetics" will mix fine. Conventional oils have a high level of synthetic content in them anyway. That is one of the first requirements of any API oil standard....that the oil be compatible with any other oil. There is absolutely no issue with mixing them.

Conventional oil and "synthetics" will mix fine. There is absolutely no issue with mixing them.

.......I stand corrected!