it’s time for an oil change - with summer on the way what would be the best weight motor oil to use and which brand(s) are good for the 4.2 lt ??? thanks !
20/50 seems to be the favorite with 10/40 a close second. I use 20/50 in the summer, personally.
Which brand? That’s like asking which beer is best. But, nothing exotic is required.
I use whatever name brand is on sale that day, or NAPA house brand.
Castrol GTX is a sentimental favorite for many. It was for me as well, for many years, but local vendors doubled the price so I said “Phooey” to sentimentalism 
Cheers
DD
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20/50 it is ! FYI Doug - Finally figured out the bumpy idle issue , it was 2 broken injector connectors , and the fix was under $50 , happy ending !
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I am running on this at the moment , you pay a lot for just a name 
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**
In a cold climate I would use 10/40 - for easier starting and good oil flow throughout…
20/50 will increase oil pressure on a ‘used’ engine, but if the oil pressure is to specs - 10/40 lubricates as well as 20/50. The latter, in a ‘tight’ engine - may not flow freely, and protect the bearings, when cold…
Frank
xj6 85 Sov Europe 9UK/NZ)
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I’m a 20W50 man - but I live in Sydney. A really cold morning for us is 6 degrees C. Paul
ok ok don’t rub it in 
Ditto in Florida. I only use 20W50.
David
But it will start fine @-15°C with 20/50. Just not too great for the engine I think.
Yeah, play to the buyers soft spots !!! some xinc is good for some older engines. Lifter vs cam shaft contact. Greenies resist it as it shortens catalyctic converter life.
But, low detergent? Dirty engine innards are good???
I track with Doug. 20- 50. Sale brand. The local Car Quest parts place sells Valvoline under their brand, At times, the local hardware store, ACE has it on sale under their store brand. I use a discount coupon ad get a few quarts.
Whenever seen, I like the 5 qt, jug. Convenient One to a change in my cars. Filter swap each change.
Although it doesn’t seem so at times, the climate here is mild. Except a few scorchers in summer. 20 50 has been my choice for a very long time.
Alas, these old bones have seen the last DIY change. Service stations are next… One offers oil and filter and a quick wash for thirty bucks. Can’t beat that… Probably a surcharge for the “odd” viscosity.
One eye doc says I’m ding better. I’ll see another for more treatment in the AM!!!
Delicious lunch yesterday with daughter. Old time BBQ joint. Emil’s. Tri tip slices, rare…
Yuvan coffee all gone. Not bad. Back to Nestle’s Classico, yum, my favorite.
Watched some You tube. Host restarting some really ancient dormant engines. Quaint “bottle fed” through the carb, over and over til they get going. Most got new points or at least got them cleaned. His personal day to day ride. S10 Chev drive train Older IHC cab and front clip. Mid fifties Ford pickup bed. Oh and an SBC for power…
Rough and ready, but demonstrates a variety of skills with simple tools…
Carl
Hi Doug , One more thought on the oil change -the xj6 has 143,000 miles on her. Would it be a good idea to add a oil treatment a( Lucas , etc.) when I do the oil change ??
cheers KO
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The engine won’t care one way or the other. Additives often make the owner feel better, though
I wouldn’t bother, personally
Cheers
DD
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Dr O!
On my 0riginal 78 XJ6 S2 with 138K miles on it I changed oil about every
2 years with 10W-40W. It never burns any oil. I never put any additives in.
In winter I just turn the key and it starts, even in 30F weather.
Walter
Amazing, but not the rule I fear…
On that note, could an unscrupulous seller add a heavy oil, like 70 w gear oil, to the engine to increase the compression numbers to fool a prospective buyer? Not sure why I think of things like this. Maybe I was a used car salesman in one of my former lives.
Phillip
Good news gentlemen , The engine is running smoother now. The old oil was black and very tired looking. The car had overheated a couple of times during this oil chapter.
The new oil is 20-50 Castrol GTX . I did not add any additives. Thanks for all you input
KO**
It was not uncommon in the old days to add even sawdust to the engine oil to quieten it before a sale, Phillip…
Thicker oil will not improve compression, but it will sure raise the oil pressure. The thing to remember is that oil must circulate to cool and lubricate the bearings. With oil too thick, for whatever reason, the relief valve will open - and the oil will be uselessly dumped back in the sump without even reaching the bearings…
Be suspicious of high oil pressure - ‘the higher the better’ is incorrect…
Frank
xj6 85 Sov Europe (UK/NZ)
**
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Saw dust was for noisy differentials.
Leaking steering boxes “fixed” by pumping in chassis lube. Actually that did no harm.
Yeah, at one time, shimming the pressure relief valve in SBC’s was popular. The design is 40 PSI and lots of volumn. The GM engineers are pretty smart…
My LT1 idles at a bout 7 psi. When revved up it goes to 40 and at driving speeds, stays at 40.
The Jeep is “stabilized” so what the real numbers are is a?? Apparently adequate, it lives on…
Carl
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