Thicker oil for hot days a good practice?


Got a Thunderhill weekend coming up. Temperatures are looking to be in the high 90s/low 100s.

I just changed my oil before my last race weekend in June. I’m running 5W-30 but wondering if I should change my oil again to something thicker – like 20W-50 – for this coming weekend. I’ve got the Mishimoto radiator, which generally seems to do a great job keeping my temps low.

Not just curious about whether it would catastrophic – I’m pretty much guessing no – but more wondering if there’s any more significant wear that might make spending the money to change oil worth it.

Any feedback would be appreciated!



One of the obsessions I went thru was with oil. I did some writing on this, years ago, in an attempt to write a primer based on what I was learning. Be advised that back in those days I was wrong a lot.

Don’t get too wrapped up into viscosity bands like xWy. What’s important is 100deg C HTHS (High Temp Shear) #'s. A high quality xW40 can easily have better HTHS #'s than a crappy xW50. HTHS #'s are available on the mfr’s website. Just be advised that oil formulations can change, and websites don’t always reflect the latest formulation.

Those viscosity’s are a “band” of visc #'s. One xW50 might be vastly thicker then another xW50. Or one xW40 might be almost as thick as a light xW50.

Heat and visc. Go look up a curve of oil visc vs. temp. You’ll find that visc changes rapidly when it’s cold, but flattens out when it’s hot. So oil visc doesn’t change much between, say 210deg and 240deg. A rule of thumb for hot oil visc is 10psi/10krpm. So at redline that’s 62psi. In a race car, don’t worry about going a little lower, but if you’re not getting 45psi at hot redline, then I’d put in heavier oil.

Oil visc drops over time. So your load or 10W40, after a couple months, is going to be thinned down to 5W30. This is a result of molecular shearing reducing the length of the molecular chains, and dilution. So when you top off, do it with oil a little heavier. Therefore don’t assume that 5W30 is inadequate for hot weather operation, it may be that it’s been thinned down to 0W20, or whatever.

The place to learn about oil is bitog,

I do not run an oil cooler and I’m in the SE. As long as there’s lots of air blast on to the oil pan, the oil temps are ok. This requires some creative cutting of the skid plate.


Just run Redline 30 WT Racing Oil year round and be done with it… I have dyno numbers and oil analysis to back this up.


I’m with Rich, Redline racing oil is great stuff. I use the xW40 tho, because I’m a wuss. For a given visc, RL has great HTHS #'s. A Redline xW30 has the HTHS #'s a good xW40. Redline is also a SpecE30 contingency sponsor for SE and MidAtl regions.


Had a crazy couple weeks up to, and including, the race weekend. I didn’t get a chance to get back on here and thank you guys for the feedback.

So, thank you! :slight_smile:

I ended up going with Mobil1 20W50 (and added a ZDDP additive). Car ran fine. No pressure/temp problems. Temp stayed around 190 (I pull temp off a small manifold plugged into the stock oil pressure switch hole).

Does the Redline 30WT have enough ZDDP to not need the additive?



That’s a lousy place to sense oil temp because it’s not in the flow.

RL race oil will have enough ZDDP. RL street car oil, I’m not sure off the top of my head. Look up it’s MSDS info.