With Pennzoil Platinum we get hip-deep into the question "are group 3 base oils synthetic?" Here I can only offer some objective info and a subjective opinion ... mine, of course!
Motor oils typically come from base fluids classed groups 1 through 5. Groups 1 through 3 are petroleum stock that has been more or less "severely refined" with such manipulations as distillation, hydrogenation and catalytic reforming (not a comprehensive list) used to improve the qualities of the base over the traditional "straight cut" (distilled and not much else) oil.
I steered well clear of Quaker State and Pennzoil "regular" oils. While they weren't bad as delivered, they used Pennsylvania crude cuts which developed a reputation for ferocious sludge formation. Poor oxidation resistance. I believe both brands use crude sourced elsewhere now for their regular oils, and the sludge problem is no worse than with other good brands of "dino" (petroleum) oils.
The premium synthetic andor semisynthetic oils from these firms are something different and better.
The USA is unique in that we can call a group 3 oil "synthetic", since arguably hydrocracking changes both the saturation level and the backbone structure of an unspecified percentage of the oil's component molecules. Imo this does violence to the term "synthetic". That said, the best Group 3s are outstanding lubricants, *almost* as good as group 4. Those are PAOs, poly-alpha-olefins, wherein every molecule (forgiving impurities...) is built up using actual synthetic chemistry. Group 5 are the esters. In ordinary usage, I cannot imagine an obvious quality difference between a top group 3-based oil and its group 4/5 kin. I found the following on the Internet, so obviously
caveat lurkor ...
Pennzoil Platinum, despite its Full-Synthetic ad copy, is largely a group 3 base. This can be winkled out from two key properties: viscosity index (a measure of how little the oil's thickness changes from cold to hot. High numbers are good) and pour point. In the 5W-30 weight, VI is 169, a very good number. Pour point is -39 degrees C, typical of the top tier of crude-based group 3.
Mobil 1, same weight, is largely group 4 PAO. Its numbers are 165 and -65 deg C. Additional factette: M1 5W-30 needs no viscosity improvers (those long gelatinous shear-sensitive molecules that allow formulation of wide weight ranges) to achieve 5W-30 spec. I could not find an equivalent statement for Pennzoil Pt, but I'd wager a similar no-or-low viscosity improver situation applies.
Objectively: six of these and half a dozen of the other.
Regarding "high mileage" formulas, my gut call is: Unless you have a proven need, fuhgeddaboudit. High-mileage oils have an additive package designed to find and plug small or slow leaks. While this is a lovely thing for engines that are starting to burn oil, seep out of their seals etc., that same add pkg is unnecessary in a sound engine and *imo* interferes with an oil's highest mission: to be as slippery as possible while providing adequate wide-spectrum engine protection. On a '70s US-made engine, 75000 miles was advanced middle age. On a well-treated Honda from the last 15 years, 75000 miles is younger'n Angelina.
The high-mileage formulae are there for customers whose protection, seal restoration ... needs are "beyond the adequate".
Finally a bit of opinion. Given the choice between group 3 and group 4, I will choose group 4. The synthetic base is slipperier than a refined-crude base which will contain some small proportion of waxes etc. that are not ideal lubricants. Also, group 4 has somewhat better stability against heat and oxidation. The difference is small, but as a hypermiler I will reach for tiny differences, then rabidly defend my choice as Morally Imperative. (giggling, ducking) But what really tips my opinion is that group 3 oils are synthetic NOT from the perspective of chemists (my former profession, although I was not a petroleum chemist, didn't play one on TV, or stay at a Holiday Inn Express last night) but from the perspective of attorneys. I could spin a diatribe on this subject spanning hundreds of thirteen-dollar words ... but I will limit myself to just one: Harrumph.
Please don't feed the attorneys...
If you've bravely and staunchly made it this far, I'll close by saying that the top oils, regardless of manufacturer and that entity's ad copy (Amsoil uses PAOs and we're the best! Redline uses esters and we're the best! We're Mobil, and there's all the rest!) use varying amounts of groups 3, 4 and 5 in their products. They all contain some polished dino group 3, if only the 10% or so used as a carrier for the additive package that oils NEED to meed, say, API specs. They typically contain a high proportion of PAO. They all contain some esters for their solvent power and seal-conditioning properties. They're all excellent products so long as you observe sane oil change intervals, use a new good-quality filter at each change, avoid gimmicks such as oils containing suspended solids (Slippery-Sixty
(tm) comes to mind...) or super oil additive boosters (made strictly from 100% organic free-range snakes). Use Pennzoil Platinum 5W-30 and go in peace, my son.
cheers apo
(edit) Wups. I just saw the mileage on your Civic. The high-mileage formula might be for you, but it seems to me cars did just fine before such formulations came out. If your engine's health, as determined by oil use, seal tighness and compression/leakdown, is tops, no need. Jmho.