Re: Rotella:
Years ago, when 'zinc' content first became an issue, I went out and stocked up before the old formula was replaced on store shelves. I am about to use up the last of it, which is likely just as well since I now am aware that motor oil indeed has a shelf life. While THAT deadline is long past, I am not overly concerned for the very same reasons that 'diesel' & 'high-detergent' are not big issues with our antique motors that are not, as a rule, heavily stressed (while possible damage from running too much detergent for 10-15 years is still not clear, I WILL say that all the 50s era sludge that came with the car has apparently disappeared, and the heads/valve gear are now 'clean-as-a-whistle', though the varnish that likely kept down gasket leaks is probably also only a memory). High-torque diesel engines are not generally subjected to the same temperatures & stress as high-performance gasoline motors, which is why the use of lower-quality/cost diesel oil is discouraged. Also, surprisingly, as it turns out, the 'newer' Rotella formula I purposely avoided actually had more zinc, not less, which was, in the end, not relevant (zinc content has nothing to do with the degree of wear protection, only it's longevity). We all assumed the new regulations concerning catalytic converters in heavy trucks would require immediate zinc reduction. Go figure.
Anyway, the main difference in wear protection between the very hightest quality oil, and one that is barely adequate, is largely only the safety margin. If you baby a lower-performance engine (don't flog it), any multi-viscosity oil that minimizes start-up wear, and is changed before the additive package is depleted/contaminated will probably do. Break-in & hard running is another matter entirely.

6 VOLTS/POS. GRD. NW INDIANA

Straight weight oils seem to be a problem to get, unless you deal with auto zone and such but at most parts stores and on the road multi vis is what were finding, I've got a newly rebuilt 312 in a 56 2 ton roll back truck, shop broke in the engine and pulled the pan to put Mummerts two piece seal in and changed the valley cover as it was leaking and I wanted the PCV valve, I ended up, with Delo 400 15/40 so I called the scrubron rep told him the vintage  I had and mostly the older stuff I've got around here he assured me that running the "diesel" oil in the gas job was not an issue.
My Union oil dealer gave up the red ball so I'll switch to scrubron oils running the 15/40 in everything just makes it much simpler.

"Could it be that synthetic oils go where other oils can't reach?" 

That's exactly the case. Synthetics didn't arrive on the scene until after engine sealing technology had advanced to to what we see today. So using synthetic in an antique/classic engine is a sure recipe for leaks unless very careful attention is paid to sealing after a rebuild. I suspect y-blocks and FEs leaked practically from day one. Old Fords mark their territory. Studebakers were notorious for oil leaks, too. Scrubbies and MOPARS not so much for some reason.

I'll add that converting crankcase breathing to a PCV system goes a long way toward reducing oil leaks since a positive system reduces crankcase pressure to atmospheric or slightly below. A passive system (road draft tube) is only effective at speeds high enough to create an area of low pressure at the tube outlet so that air is drawn through the engine.; they do nothing at slow speeds or when the vehicle is stopped.

I ran Rotella in my 292 for about 5,000 miles and began to notice yellowish waxy blobs of  goo inside the oil filler tube and also inside the cap of the road draft breather . I switched back to Quaker State. Just my opinion, but pesky little oil leaks that Y blocks seem to be known for seemed to be more profound when I once tried a high dollar Mobil 1 synthetic. Could it be that synthetic oils go where other oils can't reach?

The Rotella T-4 is rated for older non Catalytic Converter Gas and Diesel powered Vehicles.It has the higher Zinc Content which has disappeared from many Diesel rated Oil. You are correct about the T-5 and T-6 which are for later Catalytic Convertor Vehicles. Lower Zinc Content for One. T-5 is blended conventional & Synthetic and T-6 is full Synthetic which I found out  after doing a little research Online.

  

Newer diesels have catalytic converters so diesel oils are being reformulated to accommodate the change. This means their zinc and phosphorous levels are being reduced to prolong converter life just as gasoline motor oils were. Likely the "synthetic" labeling indicates that particular oil is intended for later model engines.

Most diesel oils are marked "not for use in gasoline engines" or similar verbiage. The reason is the diesel detergent package is too harsh for gasoline engines over the long term. Doesn't mean you can't use it in a pinch, it will still lubricate. But I would certainly replace it ASAP with an oil labeled for older, high mileage engines such as Quaker State Defy. And no, I don't work for Quaker State or any other oil company.

The reason I ask is that both are rated for Diesel. Reading the Label I can't see any difference other than the word "Synthetic ". Supposed to be cleaner which means maybe the Additive that we need for ours has been taken out of the Synthetic Version.



  

.Oil posts always open a can of worms. I can't count how many I have read on too many forums and everyone is an expert. Best I have been able to tell from engineering people is Diesel shouldn't be used in a gas engine ( and just forget about the zinc myth that started years ago if thats why you are using it ). Some synthetics have additives that don't play well with old type seals. Synthetic oil is just dino oil that is more highly refined. Oils that say Synthetic also say they use a carrier oil ( regular oil ) since synthetic doesn't carry the additives well.. The word synthetic is miss leading, brake fluid is considered a synthetic and it's been around for 75 years. Now they are adding this claim on the labeling, it's nothing new, just marketing BS..

I'm sure all the experts will chime in soon.

I was just going to pickup a Jug of Rotella 15w40. I noticed they have this Grade in "T-4" and "T-5" which is Synthetic. Are they both Appropriate for Y-Block? I have always used the non-Synthetic Rotella. What is the Difference?