I have an old Ford 5.8l Winsdor that presently uses Mobil 1 15w50 for oil pressure reasons. Thinking of switching to Rotella T 5w40 for the cold weather starts. Would I see a significant diff. in cold starts between the two at around 5 or 10 degrees f? What about the oil pressure when hot? Thanks

You would see much better cold flow from the 5w40 versus the 15w50, and only a slight decrease in pressure when hot. IMO.

Plus the 5w40 is a darn good oil!

Unfortunately there is only one way to really tell, and the reason I say that is because you mentioned oil pressure reasons. The 5W40 is going to be better for cold starts, and perform like a 40wt oil when hot, in an engine without OP issues there really won't be a whole lot of difference. In an engine with OP issues, the difference can be more noticeable.

Do you have a real oil pressure monitor or a dummy gage?

A real guage. When hot i get 10 lbs at idle and around 25 lbs at 2000 rpm. The max pressure I can get is around 38 lbs no matter how high the rpm. This is with the 15w50.

Consider another option: Castrol Syntec 5W-50.

I heard that it shears very quickly. Is that true?

Probably, its castrol...

FrankN can tell more about this , but doesn't the M1 15-50 flow at a pretty low temp, like -40 or so? Better than some 5-30 dino oils at that temp.

Just looked it up. -45 F.

http://www.mobil.com/USA-English/Lubes/PDS/GLXXENPVLMOMobil_1_10W-30.asp

Whats the pressure like with a standard 5w30? Just curious how bad the pressure problem is and how this thick oil is helping.

I get about 2 or 3psi at idle and maybe a max of 20 psi.

Sounds like shes needs a new pump, them old v8 fords where like that. new pump and shed be be right back to norm press with a 5w30.

I was told the exact opposite by an old time machine shop. He said bad main bearings are the culprit and suggested the heavier oil. I then called an oil pump manf. and they said a pump would not help either. The truck is a 1990 with 81K ORIGINAL MILES.

I've over the years worked around many of them older fords '84-'90 302's and 351's were all the same like that mileage didn't matter and yes new/tight cam bearings is where there pressure lies but a new pump is easer and more cost effective for old stuff like that,get your self a high volume pump and she'll be like new,trust me.....have been around it being done it does help/cure.

Do you reccommend a brand of pump