Oil per manual...
 

For the 99-04 GT's it has it posted as 5w-20 right? Thats what i put in my gt...i think...cant remember. Now im running a 03 Cobra, 15k miles, and have a few bolt ons that run it a little harder. So considering the "more harsh" conditions, heat, and normal wear and tear, do you suggest a differant oil?

I have the AFCO pro series heat exchanger to keep the supercharger a little cooler, but im focused on the engine with this. I'm going with the royal purple from AM but which weight should i go with? 5w-20 or 5w-30. Yes, i tried search with 4 to 6 words but it still came up with the "3 word minimum' thing.

Using thicker oil isnt really gonna help anything, you're already running full synthetic. If you're really worried about it, change it more often..

thanks........next answer...

I personally would stick with what is recommended. The bearing clearances are the reason for the oil weight recommendation. In my old 73 Mach, I used Castrol 20W-50, but the bearing clearances are much tighter now. I hear Royal Purple is good, but I am not really familiar with it. I am a firm believer in Amsoil.

Correct me if I'm wrong, but you could probably use any of the following and still be fine:

5W-20 5W-30 10W-20 10W-30

i'm pretty sure the 10 weight is too much. But im wondering if going with 5w-30 is too much or would hurt things. Dont know much about oil and how differant it is in a supercharged engine. Thats what im trying to figure out.

Cant offer much help except I know Modular Powerhouse recommends you dont use synthetic oil in their blown cars. Maybe give them a call?

Since you only have 15K miles stick with 5w-20 full synthetic like Mobil 1 or Amsoil. Once you get up there in miles you could switch over to 10w-30 as long as it doesn't get super cold where you live. Try not to go with 5w-30 becuase it's best to run as small as a weight difference as possible. Also, 5w-20 oil can be just as thick as 10w-30 oil at certain tempertures. You can have you oil tested by places Like www.blackstone-labs.com, cost is $22 and they will give you all the info about your oil to tell you weather you have any engine wear, what metals are showing up in your oil and other stuff to let you know whats going on in your engine. I use Mobil 1 10w-30 and my tests came back perfect after 3500 miles and two Autocrosses.

didnt know you could send off your oil and have it tested. I guess i will just stick with the 5w-20. I figured the 10w-30 would be too thick. It is the higher the first number the higher the 'thickness' right?

I use the mobil 1 10W-30 fully synthetic as well.

Both number are, its multi-viscosity oil.
Think about it like this, the second number is when its cold and the first number is when its hot. So using 5w30 would only make a difference when it was cold, and its winter time right now... so you really dont want to use thicker oil, its thicker so its harder to pump, in summer it would be more acceptable.

so your saying run a 5w-30 in winter time...thinner, easier to pump
10w-30 for sumer, since it will be warm already and "thinned" out ready to pump?

No he is saying use the oil you are supposed to run as these cars have tight tolerances and you do not need to run thicker oil for no reason. It's not going to give the car more protection.

That's false. The 1st number is the weight, the second is the viscosity. So, 5W30 and 10W30 have the same viscosity, however, they have different specific weights. Multiviscosity oil, doesn't have different levels of viscosity, just it maintains the same viscosity throughout the temperature range (or rather stays as close as possible to what it's rated).

Stick w/ 5W20 or 5W30. Stick w/ what you need to maintain lubrication and pressure. If you need thicker oil to maintain pressure, then the problem is not that the oil you're using is too thin but rather you engine needs a rebuild.

http://www.repairfaq.org/filipg/AUTO/F_oil_facts.html

http://www.blackstone-labs.com/oil_viscosity.html

My bad, I had it backwards. Late Nights ;)

But still the way that reads to me, They basically refine the oil to the first number (5 or 10) then add "stuff" so that when it gets hot it acts like an oil of the second number (20 or 30)
We're kinda getting off the main topic

ok cool. Thanks guys, i appreciate the help.

Basically you're right there, just flip it. Oil is refined to a specific viscosity and then additives are added to give it that multi-viscosity property as well as giving it a specific weight. But yes, this is OT but whatever, it's a good discussion.