Guess you can wait until winter before using 0W30 or 0W40.
I'd NEVER go back to 15W40 - it's literally a waste of energy at the oil pump.
LS engine bearing clearances are typically in the .0006 to .0008 range, which makes 15W40 just plain STUPID.
To be clear: 15W40 was a DUMB MISTAKE that may have cause engine wear in and of itself.
I like Mobil1 also, but I'm not married to it like GM is, you shouldn't be either.
Other FULL SYNTHETICS like the ones noted above are also fine options, so long as they say API SN+.
Oil consumption? No more than 1 quart between oil changes. In other words:
1. Your oil change interval should not be long enough to allow 1 qt to get consumed between changes
2. If you lose a quart, don't just add a quart back, change the oil and filter -
and try a different oil ... maybe the oil itself is to blame for being too easy to consume.
3. If different oil makes not difference, and it consumes 1 qt every 4500 miles or less, it may be serious.
Get inside the engine and find out what the 'ell is going on.
20psi at warm idle is ok, 25psi is better, 30psi is bordering on a waste of fuel.
More importantly, oil pressure should climb as RpM / engine load increases.
