Lead additive/substitute

Hello, ‘54 XK120M, S676095. Original engine as far as I know, 75,000 miles. I run exclusively 92 octane non-ethanol. This may re-release a flood of opinions on an old topic (my searches revealed no real definitive answer) but should I be adding lead or a lead substitute to my fuel? If so, any brand recommendations and ratios?


Mark in Washington State.

Don’t, no need, no reason to, definitive answer. Proper valve seats from factory.



Dont ever use ethanol fuel, so your off to a good start…
An XK engine already has hardened valve seats so does not need any lead or lead substitute additives at all, with only those trying to sell such things as saying otherwise. So just run unleaded fuel as is, without ethanol and no need for additives…
The main issue you need to be on top of is Octane Rating of fuel, and although I am familiar with our RON ratings, I am aware that the USA octane ratings use a different rating methodology so the actual octane number means different things. In Australia we get three RON ratings fuel - 92RON is the basic/lowest and is not suitable for an XK engine. 98RON is our highest, and is required for XK engines running 9:1cr pistons (and also preferred for 8:1 cr pistons). 96RON is our middle level fuel and is minimum/acceptable for 8:1 cr XK engines, but not suitable for 9:1cr.

So I think you have three grades of fuel as well, so whatever your middle level and premium level octane rating fuel will be what you want…


Great, thanks guys for the responses. Not having to do anything as far as adding something to the fuel is the easiest solution.
My 68 Datsun 510 loves it when I mix 92 octane non-ethanol with some 100 octane low lead from my airplane. I want to try that for the 120 to boost the octane a little bit. It runs just great on the 92 non-ethanol pump gas, but it’s like giving a treat to your pet.


I’ve been adding Redline Lead Substitute as well as Marvel’s Mystery Oil at every fill-up for over 20 years. When I first used the Redline stuff I started hearing occasional compression popping on deceleration (the kind you hear when you get a fresh valve job). This got me thinking that the stuff helped the valves seal better against their seats. It could also be may be my imagination, but in any case I kept using it.


as others said. the seats are hard, no need for the lead…OK…for all those that said that…agreed. gasoline back then had lead in it., it was also much “dirtier” as to deposts than now…(remember all those fuel injector cleaning issues 30 years or so ago…the fuel now has cleaner in it…). We don’t have injectors, nor catalytic converters…lead additive will not harm, if used as directed. I happen to use it.money tossed maybe…a pittance in the grand scheme of having a 120. I use a little extra “techron” or “regane” since the no ethanol fuel additive package in unknown…it is not a brand supplier. I use a little VP octane boost on hot days. Car runs great. And I like making fuel cocktails and well as my happy hour ones.

Mine is 8:1 CR and I’ve been running it on 87 octane Shell and BP pump gas for 30 years, no additives.


Thanks Nick, great information. I think the rituals of some of the things we do for our machines are as important as the physical ones.

Thanks Rob. I think I’ll keep doing what I’m doing and not add anything to the 92 non ethanol.

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… which, if the pet still has an 8:1 compression ratio, does nothing for the pet.

It might give the owner a brain woodie…:wink:

Other than loving the smell of 100 LL fuel, when it’s burned, there’s really no reason to do that to your Jaguar: also, given the specific gravity difference between pump fuel and 100 LL, you’d probably have to reset the mixture.


I have worked on cars for over 50 years, and rebuilt many of them, numbering into the thousands: I have never heard of such a phenomenon! :slight_smile: