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IMO, for a flat-tappet cam, nothing thinner than the 10w40 you're proposing changing to.

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IMO, for a flat-tappet cam, nothing thinner than the 10w40 you're proposing changing to.

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So would 5W-30 RP be ok for a roller cammed engine?

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Accord to the RP tech, you should base the viscosity you choose on oil pressure for a particular engine. For their racing synthetic oil, they recommend idle pressure in the 15 lb range, and 7 lbs for every 1000 rpm. I can't bring myself to run pressures that low, but that's what the guy said. I do believe that with the quality of synthetics these days, you can run tighter clearances and thinner oil - and make more HP. JMO.

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Like was mentioned above, the "correct" oil viscosity is a factor of oil clearance, oil temps, application (short-duration vs. endurance), etc. Spending some time on the Joe Gibbs oil web site looking through their oil application guidelines may help improve understanding of this.

I also have some concerns about using oils that IMO are too thin for extended street use, especially when it comes to flat-tappet cams. I look at things like the HTHS (High Temp High Shear) ratings for different oils because that is a good indication of how the oil should handle heavy-interface loads such as cam & lifters.

You'll have trouble finding any Xw30 oil w/ a HTHS rating much higher than 3.5, whereas you can get as high as a 5.x HTHS rating w/ heavier oils. In that respect, oil viscosity can be (like so many other things) a compromise.

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Damn Brad, you`ve got too much time on your hands. Some people love to over-complicate things and oil along w/all of the other subjects beaten to death is another one of them. I have to question how many actually try stuff instead of asking others who may not know what works and what doesn`t. I keep doing things that "experts" say can`t/shouldn`t be done again like the 8" vert on the street, spool thin oil etc. My clearances are bigger than most and even w/a high volume pump and only 6-quarts of oil, I still see 20-25 hot at idle and 65+ above 7000 rpm`s and never a problem EVEN street driving as in LOTS of desert miles and even dirt roads occationally.............just do it!

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... Some people love to over-complicate things...

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Guilty as charged!

FWIW (serious note), if you're still running Mobil 1 10w30 as I recall from the past, I really don't believe what they sell now is a "beefy" enough oil for the power level you're making. I know you're all about bang for the buck and not over-spending (I try to be, but often fail miserably at it), but that stuff is about one step above horse p!ss now from all the info I can find on it.

I've stopped running it in my '02 WRX, if that's any indication of my loss in confidence w/ Mobil 1's street oils.