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8.1 vortec. Oil weight

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8.1 vortec. Oil weight

Postby magnum man » Mon Apr 03, 2017 3:12 am

I switched to synthetic last year with 80hrs on my motor. I'm using Royal Purple. 15-40. should I go with straight 40 or 20-50? Kodiak Marine told me they used 15-40 when they still sold this motor.
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Re: 8.1 vortec. Oil weight

Postby ROCKBOTTOM » Mon Apr 03, 2017 6:05 am

1200 hours on 8.1 HO. ive never added a drop between changes,still runs like new.VALVOLINE 20-50 every 60 hours.I see no reason to pay 3 times as much for oil that your going to change as often anyway. :Drink:
take what i say with a grain of salt and a sip of tequila,I might not know what im talking about
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Re: 8.1 vortec. Oil weight

Postby magnum man » Mon Apr 03, 2017 1:56 pm

Tx sorry meant to poat ib mexhanics.
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Re: 8.1 vortec. Oil weight

Postby riverboater » Mon Apr 03, 2017 2:02 pm

I have been doing a bit of reading on this topic over the winter, I am amazed at how much power it takes to run a BBC high volume oil pump in 20-50 oil. Much of my outdated thinking I have used since my early hot rod days of the late 60's and 70's has been proven wrong. I am now dropping to 10/40 in my race engines and 10/30 in anything stock. 10/40 drops the strain on the distributor gear in 1/2 over 20/50. The high volume oil pumps are more designed for race engines that run larger tolerances than most jet boat engines. The use of a high volume pump in a tight engine just overworks the bypass and adds heat. The race folks are using 0/20 or 5 /20 in many engines on the competition circuits. I have always been a believer in thick oil and high volume pumps, keep the pressure up at about 55lbs. The last engine I built I listened to the builder and did a bit of parts swapping on the dyno to prove he had a point. Almost 25hp in a high volume pump and thick oil, and dropped oil running temp with standard pump, This was all on the dyno so in real life in a boat might be a bit different. The oil we use today has improved greatly since the 70's so use what you are happy with, the manufacturer knows his product and what oil it should take. My Kodiak 5.7 says 10/30, my mercruiser 7.4 says 10/30. Kodiak 4.3, 10/30. wife's Honda, 5/20. GMC terrain c/w 3.8 direct inject, 5/30. I think my thick oil days are numbered.
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Re: 8.1 vortec. Oil weight

Postby bigcat_ak » Tue Apr 04, 2017 11:39 pm

riverboater wrote:I have been doing a bit of reading on this topic over the winter, I am amazed at how much power it takes to run a BBC high volume oil pump in 20-50 oil. Much of my outdated thinking I have used since my early hot rod days of the late 60's and 70's has been proven wrong. I am now dropping to 10/40 in my race engines and 10/30 in anything stock. 10/40 drops the strain on the distributor gear in 1/2 over 20/50. The high volume oil pumps are more designed for race engines that run larger tolerances than most jet boat engines. The use of a high volume pump in a tight engine just overworks the bypass and adds heat. The race folks are using 0/20 or 5 /20 in many engines on the competition circuits. I have always been a believer in thick oil and high volume pumps, keep the pressure up at about 55lbs. The last engine I built I listened to the builder and did a bit of parts swapping on the dyno to prove he had a point. Almost 25hp in a high volume pump and thick oil, and dropped oil running temp with standard pump, This was all on the dyno so in real life in a boat might be a bit different. The oil we use today has improved greatly since the 70's so use what you are happy with, the manufacturer knows his product and what oil it should take. My Kodiak 5.7 says 10/30, my mercruiser 7.4 says 10/30. Kodiak 4.3, 10/30. wife's Honda, 5/20. GMC terrain c/w 3.8 direct inject, 5/30. I think my thick oil days are numbered.


I've been reading also. I called Kodiak they say 10-30 syn for up here in AK for my 350.
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Re: 8.1 vortec. Oil weight

Postby Hell Yes » Wed Apr 05, 2017 8:47 am

Mobile 1 10/30 for my 8.1 HO MP
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Re: 8.1 vortec. Oil weight

Postby Mule » Wed Apr 05, 2017 10:32 am

I agree. Run what the manufacturer recommends for your climate. They have done the R&D to determine which oil is best for their product.
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