I have used Mobil 1 5W-40 for years in everything I have with much sucess for a long time, and no sludging in the Northstars either!

Mobil 1 10W-30 Full Synthetic - SSEi w/ AC Delco filter --

No complaints, slightly better fuel economy and throttle response.

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Sort of...the Northstars have oil rings on the pistons that "sludge" and need to be cleaned if that occurs. Synthetic oil prevents this. I went almost 90K on my 1995 Eldorado using Mobil 1 every 3K miles with out having to add 1 drop in between changes. Also, the 40 weight handles the heat better IMO.

I use AC filters too on the cars, I get Isuzu filters for my Duramax, cheaper than the AC'*.

Castrol GTX 5-30...

i use it cause i had a lifter knock in my old buick but it went away with this oil but no others...

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