This is my understanding of engine oil weights. Someone correct me if I'm wrong.

Regular oil: for example, 5w-30...is a 5 weight oil that thickens to a 30 weight when it gets hot.

Synthetic oil: for example, 0w-40...is a 40 weight oil that flows like a 0 weight oil when it's cold.

Well I couldn't help myself and I just had to know, so I poured in a 12oz bottle of STP oil treatment. I know it's sacrilege, but I figured the small amount of STP wouldn't significantly affect the overall performance of the more than 5 qts of AMSOIL in the engine.

I then drove the same road under the same conditions and guess what? The increased clicking was eliminated. When I popped the hood the engine sounded the same as it normally does.

I intend to run the AMSOIL for 15k miles and change it again, but with a difference. I'll put in 3 qts of the 0w30 and 2 qts of their 5w40 Euro spec oil.

Hopefully, this will give me the same results as adding the STP, but with the benefit of 100% synthetic oil.

I think most would agree that 15,000 miles is too long an interval between oil changes especially for a car under warranty. At a minimum you need to follow what the owner's manual recommends.

No.

Oil doesn't thicken when it gets hot.

Do what I said and disregard those BIG numbers and flip the bottle around or get online and look up the numbers as "cSt at 40*C and cSt at 100*C" and you will get a MUCH more accurate picture of the viscosity of your oil at cranking and operating temps.

Why are you doing this? Now you have an oil much thicker than Nissan spec'ed for the car.

A lot of people just don't understand oil, viscosity, etc. and have been brainwashed by those big numbers on the front of the bottle that don't mean a damn thing about viscosity of that oil slinging around in an engine.

This is your Amsoil 0-30's viscosity:
Kinematic Viscosity @ 100°C, cSt (ASTM D445) 10.3
Kinematic Viscosity @ 40°C, cSt (ASTM D445) 56.56
AMSOIL - Signature Series 0W-30 100% Synthetic Motor Oil (SSO)

This is your Amsoil 5-40's viscosity:
Kinematic Viscosity @ 100°C, cSt (ASTM D-445) 13.7
Kinematic Viscosity @ 40°C, cSt (ASTM D-445) 79.7
AMSOIL - European 100% Synthetic 5W-40 Motor Oil (AFL)

This is the Nissan Ester Oil that is so highly recommended by Nissan:
MyG37 - View Single Post - Blackstone Laboratories Oil Analysis Reports

I could not find industry spec on this oil, so I found a sample with 2K miles on it sent to Blackstone Labs. The cSt at 100*C was only 8.19. So either Nissan's Ester oil (which is just conventional base-stock with an Ester addative--nowhere near as good as your Amsoil) is VERY VERY thin, or it shears down VERY VERY FAST, but considering that at 5K miles you can see that his sample sent in was 8.38, I don't think it is having an issue with shearing. IT appears to have thickened up a bit, for what reason I do not have an answer, but that is not completely uncommon.

Either way, you took a 10.3cSt oil and added a 13.7 cSt oil. When an 8.X cSt oil was spec'ed

WHY!?

For my Z (now at 9800+ miles):

Factory fill - no valvetrain clicking
Amsoil 5w30 - no valvetrain clicking
Pennzoil Ultra 5w30 - no valvetrain clicking

Disclaimer: No dogs were harmed during the above oil changes.

:)

Based on your experience it looks like I should go for the AMSOIL 5w30 at my next oil change. And that's what I will do.

I wonder just how much the STP changed the specs on the 0w30 AMSOIL.
I guess I'll change it after 5000 miles to the 5w30 AMSOIL that some others are having good experience with.

Sounds like a plan. If you want something a TOUCH thicker than the 0-30 Amsoil at operating temp, try the 0-30 German Castrol. It is a true group IV synthetic and runs around $6qt at Autozone (only place you can buy it).

Update!
 

I need to amend what I previously wrote. Just got back from a 1,300 mile flog on the interstate (@100 degrees-80/85 mph) and aggressive driving through tight mountain roads (elevation changes). Temps got up to 225-230 on that 100 degree interstate drive. Cooled considerably on the Blue Ridge Parkway (185-195, 63-74 temps). Did notice that the oilt temps would drop when engine braking/coasting even w/ the engine turning 3,800-4,000 rpm. Also found that going uphill in 4th (@2,400rpm) following slow traffic that the temps rose to 240. Downshifted to 3rd thinking that I was putting too much load on it at too low rpm and the temps remained the same; weird. Must be dependent on torque requirements. Those 240 degree temps had me contemplating a cooler, but I don't have conditions like that where I live (mostly flat). The engine performed flawlessly through this whole drive. Glad I had a quality synthetic in the sump!

Wal Mart Super Tech is a highly rated oil. :tiphat: