I’ve always run conventional 20w50, mostly Castrol GTX. I’ve been considering switching to Mobil1 15w50 or 5w50 synthetic just to see how the engine sounds with a more modern oil.
I know this topic can get very heated. Lots of myths here that people like to believe so I don’t want to get into any of that. Just want to poll what everyone is using and how you like it?
20w50 mineral always for me
I used synthetic as long as I had my '83, continue to use synthetic in my current Japanese appliances. In my experience it leaves the inside of the engine spotless, no varnish buildup. And I change it per manufacturer’s timetable, not every 3000 miles which is what I’d probably be doing if I used dino oil.
OOoohh yeah. I ran M1 Synth 15w50 for years in my 85. May have removed some varnish inside that allowed for some weeping, but I’m 100% sure it protected my engine on the track. Donald Miles of TWR & XJR15 fame will only run synthetic in the V12s he has. There’s plenty of science as to why it’s a superior product. I was probably overprotective - did filter spins at 3K and changes at 6K.
That said, people use Dino oil and that’s fine.
~Paul K.
It is, but I posit that properly maintained, for the use toy cars get, dino oil works every bit as well.
So, what are you waiting for? Give it a try and if you like it, there ya go ! 
FWIW, when I’ve used synthetic oil (which hasn’t been recently) I never noticed any change in engine sound.
Cheers
DD
I switched to the 15/50W synthetic over a year ago on my 1988 XJ-S V12, I’ve done two changes so far.
First six months, oil leaks got a tiny bit worse, but then that reversed and now they are very minimal down to just one (i just have an oil pan leak). The rear main seal looks very good.
Car runs the same and sounds the same, but as Kirby says, synthetic helps clean inside. And of course can handle high temps much better.
Its expensive having to buy two five quart bottles, but i think worth it.
My personal reason to switch to synthetic, the oil molecules are identical size. So that must help coat cylinder walls properly for rings. I switched to synthetic on my 97 Volvo 14 years ago. I’ve now got 208000 miles and compression is still in the 180s across the board.
Off topic, if you have a GM TH400, they do NOT like synthetic ATF.
I dunno where you’ll find a non-synthetic ATF.
Wha? All over the place.
And Walmart sells non synthetic Dex/Merc for only $14/gallon. I did a lot of cost research when I needed 8 gallons for that full flush!
As I understand it, all Dexron ATF is synthetic.
According to Wikipedia, Dexron historically used base 2 oils until Dexron III(G), at which point they started with synthetic base. Interesting to learn that. You guys school me in stuff I never would have looked into without reading threads.
I did switch to synthetic motor oils (Amsoil) back about 1990 for my 280Z. No longer have a Z, but still use synthetic oils.
Ditto! I learn something new from this gang almost daily!
So I’ve been putting Valvoline Dex/Merc in my TH400. Is that synthetic? Then why is the synthetic labelled Valvoline Dex/Merc so much more expensive?
I’ve been running Motul 4100 Power 15W50 synthetic engine oil ever since I bought my V12.
No problems whatsoever, no leaks, easy starting, nice cool running.
My oil of choice as well.
We all agree that snbtetic is better than standard oil. I use it in my daily driver. But, for the V 12 I use 10-40 diesel as recommended by my jag only mechanic. He explained the reason a few yrs back which if I remember correctly had to do with the rear main seal.This sounded a bit weird, did my research and appeared to be true. My car, 60K miles, no oil leaks to date. I only use it for wknd road trips, usually approx 3-800 miles with a fresh oil change when I put the car on the road usually in late April, early May. No problems to date. As a side note, for other Canadians, the Cdn Tire synthetic oil in the gold container is actually Shell synthetic. Which actually has a higher rating than Mobile. The cdn brand is approx $39.00 / 5 litres. $ saver.I, myself, will stick to the diesel.
Vic,
I am not sure who you mean by “we”, but I do not agree that “snbtetic [sic] is better than standard oil”. I have never used any synthetic oil in the engines of any of the nine classic Jaguars that I have owned. I am perfectly happy with brand name 20W50 conventional oils and following the manufacturers recommended oil change schedules in my 1969 E-Type, 1990 XJ12, and 1990 XJ-S convertible. 
Paul
All individual circumstances. I’ve been running synthetic 15W/50 in my V12 for 3 years now, runs great and leak free.
What’s more, your Jaguars are perfectly happy as well
Cheers
DD
The guy rebuilding the old caterpillar on u tube put it best. Something like “ a whole lot more engines have been destroyed by having no oil or filthy oil than by having the “wrong” oil.”