10W40 vs 20W50 in Jaguar XJ6 S1 1975

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)?

This is similar:

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.

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…

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

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.

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.

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

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