I just want to know wich is better and also if it is ok to run 15w50 in summer and 10w30/40 in winter.

 

Which is "better" is debatable.

I would suggest going with Mobil 1, 5w30 or 10W30 (on a well-worn engine).

Change every 3000 miles and watch your oil pressure.JMHO

 

I trust mobile 1 to my engine, so when I change my tranny fluid, I'll use that.

-Dustin-

 

I personally give Mobil 1 my vote because it is easily accessible and cheaper when compared to the racing line of royal purple, the street line is not fully synthetic.

how many miles does your engine have? i ran 15w-50 in 2 of my past lt1 cars that had 80-110k on them, i run a mix of 10w-30 and 15w-50.

thats my .02....

 

I vote Mobil 1, just because it's cheaper, and it's what I use.

They're most likely pretty close to the same thing.

 

Royal Purple WILL sludge up your valves if you run it constantly. Even the reps from royal purple tell you to alternate oils every 3 or 4 changes.

 

From some reading I have done on an oil forum RP street oils are not great at least not for the cost, their race oils are on par with Amsoil, Redline.

I would say just stick with 10w30M1, the M1 5w30 tends to start thin(low 30wt, range) and too thin further with use, and yes viscosity does change. Otherwise check out Patman's post on his analysis with German made Castrol Syntec 0w30 impressive, you don't need the "0" rating part but it is just a great oil reguardless.

 

That's not true at all. I know a few Royal Purple reps, and a couple of their technical reps, and they've never mentioned this at all.

I'm not a big fan of their 5w30 and 10w30, but their Racing 41 oil is very good!!

 

well if it's not true, I doubt the 4 or 5 people I spoke with would tell me that during my hour long phone conversation about why my 454 was only getting sludge only when running royal purple. They actually reccomended that I go back to a conventional oil every 3 or 4 oil changes -- I said no thanks and went back to Mobil 1, haven't had a problem since.