I just saw this on SleazeBay and it certainly looks factory made. Can anyone add some information?
The font looks like Jaguar’s font.
Heuer knows all about this kind of stuff. I’ll be interested to read his take on it.
Never seen one like that, but the admonishment to use 10/40 oil puts a certain date on it, from the 60s.
I would have said 20W50 for engines of the 60s like the XK, as also recommended today.
10W40 is a bit too thin
Jaguar had recommendations for 5W/20 oil in their owner’s handbooks. (E-type 3.8)
But no 10W/40 or 20w/50 oils are listed. Maybe a 30 point cold/hot difference was beyond the oil tech of the era. It goes 10W/30 and 20W/40, which I don’t think is even available these days. (along with a real oddball, Esso 20W/30!)
(though of course, if that filler cap is period and factory authentic, then there must have been a 10w/40, but not recommended for the E-type apparently.)
Dave
Might just be a standard filler cap (with Jaguar logo) that someone then had engraved with the oil information. The fact that the additional text is split by the Jaguar logo tends to support the idea of a modified cap.
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The Owners Handbook for the 4.2 lists Castrol XL 20W/50, Esso ExtraMotor Oil 10W/30 or Esso Extra Motor Oil 20W/40, BP SuperVisco-Static 10W/40, Duckham Q20-50 or Q5500, and Regent Havoline 20W/40 or 10W/30. Also mentioned is Mobiloil Special and Shell Super Oil. Weights aren’t mentioned so those two might have only come in one weight.
Maybe the cap was fitted to Jaguars (not necessarily E-Types, destined for colder climates.
That’s my guess.
I flipped through this collection of Mopar manuals. For 1967, they listed 20W-40, but in 1968 it was replaced by 10W-40. That’s a pretty good “not earlier” date for it.
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In an XK engine, in good shape, in all but the very hottest of weather conditions, no harm will come to the engine by using 10W-40 oil.
If it was owner modified, it was very nicely done! Almost at $50 already an unmodified one from the same seller with no bids yet!
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Since Jaguar recommended a variety of oil manufacturers, including putting 7 different ones on an E Type data plate, it seems highly unlikely that they would have a factory-produced oil cap saying to only use a Shell product. If I were to speculate, I’d guess it might have come from a racing car that had Shell as a sponsor.
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If it would fit my JeeType, I would get it!
For the JeeType, I think just using a Sharpie would suffice! 
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Sadly, not on the later style caps…
Hi, Well for the last two years I have been using Valvoline VR 10-40 synthetic racing oil. Love it, smooth running, nice oil pressure, and its Blue! Yes I know racing oils dont have detergents but with yearly oil changes and driving 2500 miles per year I don` think there is a problem. My trailer queen get 20w-50 Castrol GTX as it is still not broken in. The 65 OTS was finished in 2008 and has only been driven 631 miles… My 66 coupe is the driver.