I’m in the USA and have run 91 octane in my 1994 XJ40 for the past 18-1/2 years and 110,000 miles.
I believe your 91 is closer to our 95.
I have no idea what octane I run these days, I used to buy premium, then about 2 yrs ago switched to reg and added a bottle of octane booster, still worked out pretty expensive, so after about 9 months of wasting money on octane booster, just put regular in. No issues after 15 months of drinkin’ good ol regular.
Of course the correct answer to what grade of fuel you should use in your car is to follow the manufacturer’s recommendation …
..... BUT ....
The manufacturer is always going to recommend the grade of fuel for the most extreme driving conditions their car may encounter. So if you are a “boy racer” who redlines the RPM at every shift and drives around with the gas pedal always on the floor that’s makes a lot of sense. Also not a bad idea if you’re pulling a trailer up Mt Kilimanjaro.
The 3.6L engine came in two variants.
In North America you can check the 7th letter in your VIN:
“5” … 3.6L Low compression 8.2:1
“6” … 3.6L High compression 9.6:1
My 89 3.6L XJ40 owners manual recommends premium fuel 91 AKI (95 RON). But since I have the low compression engine and admittedly drive like an old lady I’ve felt totally comfortable using regular fuel 87 AKI (91 RON) for the past 27 years with absolutely no adverse effects. I recently performed a compression check on my engine (that runs smooth as silk) and it was spot on
So to each his own. Remember, running higher octane fuel than necessary won’t hurt your engine, just your wallet.
Allright so the fuel is fine lol.
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Thanks groove man,the myth had me fooled for years,lol.i will be happier filling up with 91 saving $$$. Alexandre have you found the problem yet to your cold start issue, sorry, didn’t we get a little side tracked 
. definitely not the fuel, unless it’s old and stale sitting for a long while or contaminated.
Oh, I to was in the Navy, though kept my feet planted on deck, had the experience of exercising with USS Enterprise , fond memories.
Gordon … Always good to hear from a shipmate 
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Sounds like may be multiple problems…
First:
OH and just to be sure… replace rotor even if it is new!!.. I had a rotor short to the center metal distributor shaft cased all sorts of odd no start conditions…
Then check for good spark cranking… pull plug wire insert metal toughing plug connector and bring near ground … crank engine… good spark?
If not same check at coil (disconnect coil center tower wire from distributor…check spark now its good sharp blue spark?? If spark is different bad cap or bad rotor…
Another high voltage test… do what you can to get car started at night… with car running spray-mist water on all the high voltage components… you might not have to even do this to see blue high voltage leaking and arcing to grounds… would indicate bad dielectric = to bad wires bad cap bad coil tower bad rubber boots–could be any or a combination of voltage leaks… will be a visual display of any high voltage leakage…
good luck
Hats off to you, now I understand “Grooveman”- Tomcatters flying Tomcats, VF14 flying the F14- coincidence? Probably not.
I’m thinking that the engine management system may not be getting a correct reading of coolant temperature thus the problem in starting when cold but not when hot. There are two coolant temp sensors one for the instrument panel gauge and one for the engine management system. Are you sure you changed the correct one? On my 1990 4.0 liter the one with the square brown plug is for the engine management system.
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