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Thread: Motor oil grade

  1. #1
    FEP Senior Member Sask84gt's Avatar
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    Default Motor oil grade

    Hey guys, I know this topic has been done to death but I had a question regarding oil grade. Motor is a stock 91 302 HO with 200,000+ km. Does it matter if I go from 10w30 to 10w40? I originally put Castrol gtx high mileage 10w30 which is a synthetic blend oil but I’ve noticed over the past year it has developed more leaking and I’m wondering if that’s due to it being synthetic. I went out and bought some Castrol 10w40 conventional oil and thought about switching but now I’m not too sure if I should. Oil pressure is excellent using the synthetic blend but I figured maybe a heavier oil that’s not synthetic may help leaking and be better in summer with the higher heat. Will it hurt to switch or should I go with 10w30 conventional instead of blend. Or am I way overthinking this(most likely) LOL
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  2. #2

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    Give it a shot.

    I think you’re definitely over thinking it (I do the same).

    The owners manual to my car says 10/30 and 10/40 are both recommended.
    1986 Mustang GT - 2A, 5 Sp
    2018 Subaru WRX (Daily)

  3. #3

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    Throw in some of the high mileage 10-40. Most of them have 'seal conditioners" in them to get old seals to swell a little and seal better.

    JIM

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    Moderator wraithracing's Avatar
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    My $.02 is I would run 10w30 in the colder months if driven and you can run the 10w40 during the warmer months if needed. If oil pressure at full temp and during idle is above 30 psi, then I would consider running the 10w30 during the warm months. No need to put added stress of high pressure on a high mileage engine. The High Mileage oils do have additional additives to help with seals, etc. so not a bad idea, too.
    ​Trey

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  5. #5

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    I like a lower weight oil for more oil moving around, if you have good pressure I would stick with lower weight and dino is fine just be good about changes. If you are not racing and doing high engine temps where oil breakdown is an issue, no need for synth
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    FEP Super Member webestang's Avatar
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    I have run nothing but Valvoline Durablend 10w30 (MC FL-1A filter) in my 88 5.0 since I bought her in 2002. I too have 200,000 miles on stock, untouched, non-raced engine. Drove it for 10 years, year round in the beginning and have not a single oil related issue.

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    FEP Power Member richpet's Avatar
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    Quote Originally Posted by wraithracing View Post
    My $.02 is I would run 10w30 in the colder months if driven and you can run the 10w40 during the warmer months if needed. If oil pressure at full temp and during idle is above 30 psi, then I would consider running the 10w30 during the warm months. No need to put added stress of high pressure on a high mileage engine. The High Mileage oils do have additional additives to help with seals, etc. so not a bad idea, too.
    Agree. Personally, I always use full syn oil and bump my changes out to every 6 months/5k give or take. It is easy to overthink though. Lol.

    The high mileage oils do have more 'stuff' in them that can benefit an older (or flat tappet motor which you don't have) motor. My toy car gets high mileage 10/30 year round.

    I bought my truck ('03 5.3 Silverado 4wd) a couple years ago showing 186k and recommend 5/30 oil. I immediately switched to syn and do similar to what Trey mentions. It has 5/30 in it now, will get 10/30 it's next change. It will be towing season after all.
    I am at 205+k now. Zero leaks/noise or anything to make you think it has over 50k on it.

    It boils down to:
    Run a good quality oil you change at an appropriate interval.
    Run the lightest oil that does the job well.
    Even Wally-World brand syn is 'better' than dino.
    In your motor? I'd go 10/30 syn year round, or go for the diesel 5/40 T6.

    The one thing about syn to keep in mind - it does a better cleaning job. A high mileage motor probably has lots of small seal related leaks that are simply ughed by gunk. The syn can clean that and now you see the leak. The high mileage versions do help combat that potential. Every motor is slightly different.

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    Last edited by richpet; 01-14-2023 at 12:26 PM.
    83 5.0 GT. Quicker than it looks! 10:1 (or just over) 306, Motorsport a332 cam, 140A alt, t5 conv, 8.8 w/ 3.27's, Edel rpm, alum rad, very worked e7's, Holley SA carb, etc... SOLD IT!!!!

    Now an 1981 Granada! .040 over 302, Edel E-street heads... Currently building a 347 because, why not?

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    FEP Senior Member Sask84gt's Avatar
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    Thanks for all the replies guys. I think I might just stick with the Castrol gtx high mileage 10w30 synth blend. Car is only driven in the summer months and I’ve got good temps and oil pressure so that seems to be the best option.
    Mustangs
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    85 Gt

  9. #9
    FEP Super Member gr79's Avatar
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    Currently am sticking with Valvoline 10w30 conventional in the Mustang's turbo engine.
    Last read is 1/2 qt down after 2500 miles. Better than when i bought car in 1980.
    Valvoline 5w30 Blend in the truck since 'new' at 5k and nothing else.
    Minimal amount of oil use looks to be from oil pan area leaks and turbos.
    Have used synthetic in both engines. No advantage noticed.
    FL-1A filters and new drain plug gaskets if on hand. Both have magnetic drain plugs.
    Changed every 5k or yearly. Both engines have well over 100k miles.

    Am not sold on himile oil anymore Seems to cause non dripping oil pan gasket and seal seeping.
    Have blue Felpro permadry v/c gasket on Mustang engine. Truck has factory gasket. no leaks. Both are 2.3L

    Have used other brands. Ok, but not sure of additive packages in them. WM, store brands, Mobil 1, NAPA, VR-1.
    Have owned cars since the late 60's. Hundreds of oil changes, prefer Valvoline brand.
    Castor GTX semi oil should be ok.

  10. #10
    FEP Power Member richpet's Avatar
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    Quote Originally Posted by Sask84gt View Post
    Thanks for all the replies guys. I think I might just stick with the Castrol gtx high mileage 10w30 synth blend. Car is only driven in the summer months and I’ve got good temps and oil pressure so that seems to be the best option.


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    83 5.0 GT. Quicker than it looks! 10:1 (or just over) 306, Motorsport a332 cam, 140A alt, t5 conv, 8.8 w/ 3.27's, Edel rpm, alum rad, very worked e7's, Holley SA carb, etc... SOLD IT!!!!

    Now an 1981 Granada! .040 over 302, Edel E-street heads... Currently building a 347 because, why not?

    "Never underestimate the power of stupid people in large groups"

  11. #11

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    Amsoil Signature Series 5w-30 last fall for me, probably put 200 miles on it.

    I have fresh Amsoil Signature 10w-30 and filter waiting for me to do in the spring.

    Probably wasteful, but I’d rather get top tier oil and have it be a placebo than worry about splitting dimes and effectiveness.

    It did leak a few drops out of the front drain plug, but I attitude that to not having a fresh crush washer. I’ve got a set of new plugs and washers to use next time, with the thicker weight oil.
    1986 Mustang GT - 2A, 5 Sp
    2018 Subaru WRX (Daily)

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