10W40 vs 20W50 in Jaguar XJ6 S1 1975

XK-Engine
#1

Good day to all!

I use a XJ6 motor from the 70s in my self built car, which weighs approximately half of the XJ6 and never sees much above 120 kph (give or take 3.000 rpm).

So I figured my operating temperature is a conservative 100°C (I know, I need to validate this). Since 10W40 lubricates better than 20W50 at lower oil temperature, I expected a benefit.

However, the oil pressure dropped from 40 psi to 30 psi at cruising speed. This does not have to be a bad thing, but I am curious.

The pressure values are also not validated (I read off the stock, 45-year old gauge), but the drop is a fact.

I have a few next steps, from replacing with 20W50 to reverse the effect, via an actual temperature check to fitting a calibrated pressure gauge.

But I am lazy and like to hear an opinion about the drop in psi.

Has anybody run their XK engine with 10W40 (or 0W40)?

#2

This is similar:

#3

Another alternative you might consider is to use a synthetic oil. I use Mobil1 15W-50 synthetic.

Whether it makes a meaningful difference in engine temperature depends on which internet source you read. As it does reduce friction I’d expect some temperature reduction. How much, I don’t know.

Gauges of verifiable accuracy are always useful in diagnosing a problem. I removed the temperature sending unit from the car and suspended it in a pan of water on a hotplate beside the car. I had two kitchen thermometers in the pan. I extended the signal wire from the cars wiring harness to the sending unit and also ran a ground wire from the body of the sender to a ground on the car. I turned the ignition switch to the run position (don’t start the car) and turned on the hot plate. I monitored the temperature of the water in the pan and noted the corresponding position of the needle of temp gauge.

Now, at least I know what the temperature is when I look at the gauge.

#4

Dominik, I would stick with 20/50 overall but if your going to use it all year round we used to cheat with 10/40 in REAL COLD weather for faster flow.
Others suggested Synthetic, We went down this road…
Even if completely rebuilt the tolerences are to sloppy compared to a modern motor.
You will leak more oil than 5 holes in a boat…
IMHO.
GTJOEY1314

#5

Nothing to be too concerned about: it’s not so much the pressure of the oil, but the flow.

Generally speaking 10 psi per thousand RPM is a good rule of thumb, and if it’s close to that, you’ve got nothing to worry about

#6

Thanks guys!
My objective is to run the oil at its designed (or close to) optimum operational viscosity.

If my engine runs at 100°C, this is closer to the W40’s range to get 10 cst. The W50’s oil optimal temp would be higher.

I like the 10 psi per 1.000 rpm guideline. Heard it before. Let me drive the car at 4.000 rpm and 5.000 to see the increase. That should be easy :wink:

Unless the bypass is set at 40 psi.

#7

It’s fine and if it starts to leak use 20/50 again. If it gets very hot where you are the 50 is probably better, at idle, but if that drop is not alarming, oil is oil.

#8

Not to sweat it: if you have anything from 30 and up, you’re golden!

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