Hi Dave03S, not sure really, I have been a big fan of top quality oil and I think synthetic stuff makes sense with the cold starts we get up here. I use liquimoly with zinc additive in it. The PO was using the regular castrol fills from the dealership at 12000km and he sometimes went a few years without a change as far as I can see from the receipts that came with the car and I believe that might have contributed to the lifter wear as there was coked up oil deposits on the sides of the lifters. The oil debate is always a good one but I can't say I have facts to back up my choice to use synthetic. Any specific reason not to you think?

Yeah I don't want to get the old oil debate going again here in your thread, sorry. I follow the belief that these are old designed loose tolerance engines. The molecular structure of synthetics is designed for tighter tolerance newer design motors. While the weights might be the same the intended use is different.

But mostly I shy away from them because when I got a factory fresh replacement 4.6 the dealer broke it in with synthetic and it consumed a quart of oil every 900 miles for 160k miles even after I switched to conventional oil.

And because both of the highly regarded independent LR shops I frequent use conventional 20w50 in all of their customers Buick 215 based V8's. I trust their decades of knowledge on what works best.

Conventional oil will cook off to ash, while synthetic is still fluid and working normally.
Being an older pushrod design is no reason not to use synthetic.
The only arguement against synthetic in a 4.0/4.6 that I have seen is that it tends to leak past the cruciform seals.

How long do you have to leave it in there to cook off to ash? Seems like any normal 3k oil change would prevent that.

Another issue is like the PO said, people put synthetic in and don't think they have to change the oil for 12k miles.

Maybe in a Toyota but not in one of our trucks.

Yes, normal oil changes prevent that, but synthetic doesn't break down from heat like conventional does.
I always use synthetic...but put Rotella in my D2.

I could be wrong and I don't care, but I change the oil every 3k and I use cheap conventional oil.

I was told that synthetic oil maintained its molecular shear strength ability for 6,000+ miles, whereas conventional oil begins to slowly lose shear strength (the ability of an oil molecule to resist metal edge to metal contact/pressure) at 2,500 miles and conventional oil is dang near useless for preventing internal engine wear by 5,000 miles.

Like I say, I don't care if the above is wrong, that's what I go by. Pretty minimal wear to be expected changing at 3,000 miles each time, and the conventional oil is cheap.

No need to worry about what leaks out needing replacing or what burns off needing replacing because the conventional oil refill is so cheap whenever needed.

Totally agree with the change intervals.... I use synthetic but I still change at slightly above 3000 miles (5000km, easy round number here)
Most likely overkill but it's cheap insurance I think as I do the oil change myself.

This is my findings with my 928's. One's a 78 and the other is a 84, if I use synthetic oil, any weight configuration...they leak like the Exon Valdez. Dump old school oil in...not a drop. Also, oil with a zinc content was crucial with them. Flat overhead tappets would clatter like a mother on start up with synthetic.

Brian.