Yet another oil question - mix SAE 30 with 10W-30 ?

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Northstar Engines and System Technical Discussion Discussion, Yet another oil question - mix SAE 30 with 10W-30 ? in Cadillac Engine Discussion; As the weather becomes cooler, I'm just about to change the oil in the ol' 94 STS. I've come across ...
  1. #1
    scott f's Avatar
    scott f is offline Cadillac Owners Fanatic
    Automobile(s): 2002 STS/ 1994 STS
    Join Date
    Oct 2010
    Oshawa, Ont. Canada

    Yet another oil question - mix SAE 30 with 10W-30 ?

    As the weather becomes cooler, I'm just about to change the oil in the ol' 94 STS.
    I've come across some 10-15 yr old sae30 API SL,SJ,SH,SG motor question is can I use 1l of this with the reguarlly prescribed 10w30 Castrol GTX as a zzdp backup? Hope this makes sense as API SG/SH is whats on my oil filler cap
    If this is a good formula I would use 6l of Castrol GTX 10w30 and 1l of SAE30.
    Does this sound good for the ol flat tappets?
    P.S. 296K km
    As I start to think about it...what the hell am I doing?
    I've made it this far what's the difference?


    I bought this '94 STS in 2000 with 153K km and didn't have a clue about this forum... just changed the oil with whatever was on sale and 1 bottle of Lucas oil stabilizer,always overfilled! Guess I did something right

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  3. #2
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
    Automobile(s): White Diamond '03 DHS (with DTS floor shift)
    Join Date
    Jun 2004
    Woodstock Ill.

    Re: Yet another oil question

    My understanding is that you do not mix straight weights and multi-grade oil. You can mix straight weights like SAE 10 and SAE 30 or multi-grades like 10W30 and 10W40. Run your straight 30 during the summer if you want to.

  4. #3
    Submariner409's Avatar
    Submariner409 is offline If it won't run, chrome it
    Automobile(s): 2002.5 F55 STS - 67,500 miles, 2014 Explorer XLT FWD
    Join Date
    Feb 2006
    MD Eastern Shore, Kent Island

    Re: Yet another oil question

    Before I would run straight 30W in a modern engine, particularly a Northstar or other close-tolerance engine, I would do some talking to big-time engine builders. The multigrades were developed to provide proper, quick cold/cool engine lubrication. I won't even go into cold oil and filter bypass valves.............

    As Ranger posted, mixing straights and multigrades is not a good idea - there's a lot more to consider than simply the end viscosity.

    Want to spend some money and properly put some extra ZDDP into your oil fill ?? Online a case of JoeGibbsRacing HR-4 synthetic oil and use a quart with the other 6.5. Or look into one of the 10W-30 HD oils from Chevron, Gulf, Pennzoil, Shell - and you may find that they carry an automotive SG or SH rating due to their older ZDDP package level.

    Leave the Lucas stuff on the store shelf. Yes, they're a big company, yes they advertise well, yes they have stickers on NASCAR, and no, I wouldn't touch any of it with a 10 foot pole. Same applies to another well-marketed additive/cleaner chain. Ask GM if they recommend additives.

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