Mr.T

3.8L oil pressure comparison

5w-30

• Idle == 16 PSI
• 2000 RPM == 47 PSI

5w-40

• Idle == 18 PSI
• 2000 RPM == 52 PSI

5w-50

• Idle == 22 PSI
• 2000 RPM == 55 PSI

Oil was ~200F measured with an infrared thermometer on the filter. 2000 RPM is cruising at ~55 MPH, 60F ambient, then idle oil pressure and filter temperature were recorded. It takes quite a while longer than coolant/engine temperature for the oil pressure to stabilize. The pressure at higher RPM's and throttle depend on the duration, ambient temp, engine temp, etc. I'm seeing roughly the same pressure at full throttle and 4-5K RPM as the 2000 RPM cruise on a long steep grade, and the pressure obviously stays lower across the board until the oil cools back down. The oil pressure relief setting is about 60 PSI based on observation, but the spec is non-existent (not in the FSM, and even a tech at Melling couldn't find the number).

DEJK2012

Mind me asking, why dont you ever test 5w-20? I have put it in the Jeep before (not on purpose). I only ran it until i realized what i did then put 5w-30 back in it, but i ran it for maybe a thousand miles before i noticed. Now before people start attacking, i always keep spare oil in the garage and have like 4 different weights sitting around. But i grabbed the wifes trucks oil and not the one for my jeep (both royal purple and was hot and wanted to get it done with that day before we left on a trip).

Mr.T

No real reason for not doing a quick test other than time. From looking at the other data, the pressures would likely be around 3 PSI less than 5w-30 at idle and 2000 RPM cruise -- But can't say for sure without testing. Will keep an eye out for some inexpensive 5w-20...

On a related side note, I'm using DEO 5w-40 in just about everything (tractor, mower, pick-up, mom's car, diesel/gas etc.) rather than stocking lots of oil in the garage.

Mr.T

Sample #12 at 122500 miles using 5w-40


Still running great, no internal engine repairs. There's about 8 years and 110K miles in 12 samples in the graph above. Regarding the spike at sample 8, the washable air filter is the prime suspect for the higher silicon (from dust) causing the higher than normal iron, chrome, and aluminum (see posts 147 and 149). Not knowing when periodic air filter cleaning has degraded air filtering ability, especially a Jeep that eats a lot of dust is a deal breaker.

Next up is the Amsoil (EFM) Euro 5w-40. There's been a lot more short-trip and daily-driven in the last few years, plus a shorter OCI is fitting with my schedule. EFM about $2 cheaper per quart than DEO, and is light duty diesel rated in addition to the various Euro and US gasoline ratings (SN). The higher detergent of diesel rated oil might keep the rings and lands cleaner, which I'm thinking is a sticking point (pun intended) with these engines. And since I'm not going for the long OCI anymore, EFM looks more appropriate. EFM is also on the lighter side of the grade 40 viscosity range.

08 Steel Blue

Hi Mr T, I hate to keep asking you questions today, but some of your posts intrigue me. As I have read through this thread and see the various viscosities you have had analyzed, I was wondering since the last post on this thread was 4/29/19, have you drawn any conclusions as to which viscosity has been optimal for your 3.8? Thanks!

Mr.T

5w-40 is my favorite. Readily available at a decent price in synthetic. I've one or two more oil samples to add to this thread, basically just more of the same though...

08 Steel Blue

Thanks! I have been using the Castrol Edge 5W40 European with good results as far as oil consumption goes, but have been tempted to try amsoil 5W40 and the Rotella 5W40.

Mr.T

It's been ~9 years, ~140K miles, and 14 oil samples!

There's been lots of camper trailer towing this year, and the next sample #15 (158300 miles) may be real interesting when the results come in (next week perhaps). The oil pan and transmission case have temperature indicators added that trap their respective maximum temperatures -- And they both hit new highs this summer while towing long steep grades in ~115F temperatures. The oil pan hit 250F and the transmission case showed 230F. However, these are external metal temperatures so the oil is actually hotter, plus the temperature increments are in 10 degree steps, so I'd estimate the fluid temps are at least 20 F higher than indicated by the surface temperature -- Engine oil at 270F and the manual transmission oil at 250F. To paint a better picture of the circumstances: imagine towing a camper up long grades at full throttle, 3rd gear, ~4500 rpm, and ~115F outside with the A/C on. Sounds brutal, but it ran great. Maybe later I can find a chart to post that shows the viscosity lowering as temperature goes up, and comparing various oil grades (e.g. 5w-40, 5w-20, etc.).

Mr.T

We're up to 158300 miles with sample #15. There was a lot of towing this OCI, about 3500 out of 5300 miles. Much of it in hot weather, including long steep grades at full throttle in the 4000 to 5000 RPM range. Oil consumption at 3000+ miles per quart remains in the same ballpark, but seems to be improving during the latter half of this OCI.



The oil pan and transmission case have temperature indicators added that trap their respective maximum temperatures. Engine and transmission oil both reached new all-time high temperatures -- The oil pan hit 250F and the transmission case showed 230F. However, these are external metal temperatures so the oil is actually hotter, plus the temperature increments are in 10 degree steps, so I'd estimate the fluid temps are at least 20 F higher than indicated by the surface temperature:
Engine oil at 270F and the manual transmission oil at 250F.

With all that in mind, does oil analysis show any increase in wear metals compared with previous data?

Nope, it looks pretty much the same.

RLT

Having a manual how often do you change the oil in the gear box.
Change mine every 2 -3 years regardless of mileage.
Still using Moble1 0w-40 in my 2.8CRD and at 107,000 appears still not to loose any !
Apart from when the oil pan seal went bad.