muzzo
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- Jan 12, 2020
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- Corpus Christi
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- Vince
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- 2019 Bullitt, 2017 F250 KR
In South Texas summer heat is 100+, with heat index’s of 115 to 120. 5w 30 still works fine.Bearing clearances are what establish the viscosity required, primarily. Tight clearances = low viscosity oil, loose clearances = heavier oil. It's not a bean counter decision, it's a function of the design. And of course there's more to it.
Improved modern production tolerances have allowed OEMs to use tighter bearing clearances and lower viscosity oils which help in efficiency and hitting fuel consumption targets. Counter-intuitively, the tight clearances also increase the load capacity of the bearing for a given journal diameter, but the decreased oil film thickness leaves less room for error. Higher than normal cylinder pressures due to boost or nitrous demand an oil with higher film strength to maintain bearing capacity without collapsing the oil film thickness to zero. Very high cylinder pressures at relatively low RPM is the main reason diesel engines use 50 weight oils along with larger bearings and clearances. For a gas engine, basically don't spray on the hit unless your motor is built for it, or you'll quickly introduce your rod bearings to your crank.
This brief article explains this relationship well: https://blog.k1technologies.com/bearing-clearance-and-oil-viscosity-explained
Mechanical shear of the oil film and heat both play a role in diminishing the viscosity of your oil. This is where the quality of your oil may be more importance than the viscosity.
I've had a half dozen cars now with OE or aftermarket forced induction, so I've done a lot of reading on oil and here's my $.02
- For an OE F/I application I use Mobil 1 in the recommended weight. It's cheap and really good. If boost is increased from stock I'll go up a bit in viscosity. My Audi regularly sees 20-22+ psi so I use 5W40 Euro instead of 5W30. Engines built for boost will typically have larger bearing clearances and a higher flow oil pump to suit. The shear forces on the oil film are lower which reduces oil viscosity loss due to mechanical shear, so the need for an oil with exceptional "shear stability" is not as great. The trade off is needing a larger journal diameter to maintain the bearing load capacity and reduced efficiency.
- For an aftermarket F/I application I use Amsoil and go up a step in viscosity. A naturally aspirated engine will generally have tighter bearing clearances than one built for boost. Amsoil does have higher shear stability than Mobil 1 so you get better protection without having to go full race weight on your oil. 5W30 Amsoil Signature is a great all-around option for a boosted Coyote street car even if it sees occasional hard use.
- The biggest reason to go up to a 50 weight for a healthy gas engine is extreme heat. If it's truly a track slut, then go for the 5W50. There's a reason the GT350 has an oil temp gauge in the dash (although I think it's a derived value, not actual). I've got a lot of experience with air/oil cooled bikes and they all require heavy 50 or 60 weight oils because of the heat. If you've ever cooked a big twin in parade traffic then checked your oil you'll know it's as thin as water, maybe less.
So will it hurt to put 5W50 in YOUR car? Probably not, but bear in mind that all else being equal, heavier weight oil will yield higher oil pressure, but lower overall flow. Oil has 3 jobs - to act as an oil film bearing surface, to provide cooling, and to provide lubrication - in that order of importance. Your 50 weight might improve oil film strength, but at the detriment of cooling and lubrication which depend on flow. Unless your operating temps are up high enough to warrant it, skip the race weight oil and stay closer to what the manufacturer recommends.
/nerdfest
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