What oil to use??

I have a '93 Mustang GT 5.0 HO with 97k original miles on it. It runs great and the oil pressure is good, but could be better. I am currently running 10W-30, but am wanting to thicken it up just a tad. I am weary about going as thick as 20W-50, for fear it might damage the oil pump. I wanted to use 15W-40, but it is only recommended for diesel engines. Why? What would everyone recommend for a thicker oil? I live in N. TX, so it doesnt get below zero here.....haha.

i use Mobil-1 synthetic oil, in my 96 mustang with about 76k on it, and its treating me good.

Conventional oil is fine as long as you keep with with your 3k mile/ 3 month oil changes. I an running 10W40 right now, and during the hot summer months, and 10W30 goes in right before storage in the winter. 20/W50 might be a little much, unless your all out racing the motor when it sees excesive heat and constant use. 20W50 will also make the car run slightly rougher, and eat up a few HP due to the thickness of the oil in the crankcase.

Thanx guys, but it still didnt answer my question on why is it not feesable to run 15W-40 in anything but diesel? And what do you guys think about a single grade oil, such as 30HD? If I cant run the 15W-40 I want, then I will probably take your advise and just run 10W-40, just to make it a tad thicker than the 10W-30.

not sure about the desil thing...
just try some some red line or royal purple.. i hear there the best, then u can tell me what u like so i can use it in my car and not waste my money.. :loser: .. and use 10W-40..

<---- uses 0w30

A 5.0 with that many miles will be fine with 30 or 10w30. I will not use anything but synthetic in a performance app. Mobil1 is my choice. It is easy to find. If you are using non synth, I recommend Castrol. You definitely don't need a multi-viscocity (ie 5w-20) in the warm months. There are other things added to a standard multi-viscocity oil. These additives give you more impurities compared to a standard weight. The 10w-30 Mobil1 I'm using seems to burn and drain very clean. I just pulled the covers on my 35K motor. Not a single small piece of crap anywhere. The cast iron was still slightly shiny in fact.