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It seems there have been a lot of oil questions lately, and I am going to add one more.

After the rebuild of my engine, I used synthetics. I am currently using Amsoil 10w-30 Synthetic with an Amsoil Filter. I plan on changing to Castrol Syntec as I have no local Amsoil dealers, and I must order online.

While playing around on the Castrol web-site, their system says that I should be using 5w-20 in my 4.9L. I emailed Castrol, and they said that this is what ford recommends, and they said they would not advise varying from it.

I live in Southern Georgia, and have always used 10w-30 in most of my vehicles.

This next go around should I change to 5w-20? Why or why not?

Ford suggests it for a reason I guess.

Thanks guys! :banghead
 

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I live up in Statesboro and use 10w30 Motorcraft 1/2 synthetic or partial synthetic or whatever the heck it is in my 351w & 302 and use the same brand 5w20 in my 4.6. It's just what the oil cap/owners manual called for so I used it. Don't have any problems with either weight.
 

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A 4.9 is most DEF. 10W30. I live in Statesboro, and that's what goes in my truck, as it's what the manual calls for.
 

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I use amsoil synthetic as well. The manual says 5w30 is preferred but I have always used 10w30 and have never had a problem in the last 12 years. Doesn't use a drop and runs like a top.
 

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Why does my '89 call for 10W40? That's what I've been using but I want to know if I can run 10W30 in my rebuilt engine.
 

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Why does my '89 call for 10W40? That's what I've been using but I want to know if I can run 10W30 in my rebuilt engine.
I'd ask my engine builder if I were you. It all comes down to what the tolerences are inside the engine.

95F-150: I just looked on the AMSOIL site and it says to use 5W-20 there too. That's strange since I don't think that was available back in 1995.
 
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