All,
Apologies that my time to browse the forums and address questions such as all those above is not what it was 15+ years ago when I bought my 02 Z06, immediately started tracking it and began racing it in 2003. I do respond daily to PMs and emails, normally after hours when I get home from my "day" job. My race team (and associated AMSOIL dealership) are very much a hobby.

Will try to address all the above questions, in no particular order.

C7 Engine Oil
The proper oil as indicated in the Online Application Guide is the AMSOIL Signature Series 0w40

The reasons why are complicated. When GM changed the recommendation for the C7 from M1 5w30 (a dexos 1 oil) to M1 0w40 (dexos 2), in my opinion they really muddied the waters with regards to oil specifications. Broadly, there are two main bodies that provide oil specifications, the American Petroleum Institute (API) in the U.S., and the European Automobile Manufacturers' Association (or Association des Constructeurs Européens d'Automobiles in French (ACEA). These two organizations don't always agree on what oil specifications should be as engines, fuel and emissions systems vary by country. Further complicating the oil specification situation is the trend of the auto makers to release their own specifications, like GM's dexos 1, dexos 1 gen 2, and dexos 2. This challenges the oil companies as it becomes almost impossible to make one oil that meets all of these standards, so they end up making many variants complicating their manufacturing and supply chains. Even further muddying the waters, both the API and many companies (like GM) charge oil companies to test and certify their oils to these various specifications which is extremely expensive for both the testing and paying the licensing fees. To further generate income, in general, if an oil company changes a supplier for some ingredient (basestock, additive, etc.) in the oil, they must pay to retest and recertify.

I haven't looked at a C7 owners manual after the change in recommendation to the M1 ESP 0w40, but prior to that and dating back at least to my 02 Z06, the owners manual stated that the oil must meet both a GM standard (then 4718M superseded by dexos 1) AND API specifications (then API SM superseded by SN, then SN Plus). The dexos 2 specification is primarily intended for emissions systems that have particulate filters (e.g. diesels), but isn't specifically a gasoline or diesel specification. The M1 ESP 0w40 meets this spec, but it does not meet any API spec. I have not researched how GM/M1 can say that this oil is backwards compatible when the oil doesn't meet the standard listed in prior owner's manuals (e.g. API).

When GM changed their recommendation for the C7, I informed AMSOIL and they looked into which one of their oils best met GM's somewhat conflicting specifications (API going backwards, dexos 2 going forwards) and the viscosity requirements levied by GM (e.g. 0w40 vs. prior 5w30). The best two fits were the AMSOIL Signature Series 0w40 and the AMSOIL European Improved ESP 5w40. The Sig Series 0w40 meets the new stringent API SN Plus spec designed for the challenges of TGDI engines where Low Speed Pre-Ignition (LSPI) is a major problem and is the proper viscosity range. The Euro ESP 5w40 meets API SN (but not the newer more stringent SN Plus) and dexos 2, but isn't the correct viscosity. After completing their analysis of all the factors involved, AMSOIL updated their product application guide to recommend the Sig Series 0w40, which is the change noted by several of the above posters. The performance of the Sig Series 0w40 and Euro 5w40 is extremely similar at the top end (e.g. hot or hotter). The main difference is that the Sig Series 0w40 will be better when cold or in cold environments, and it has a much longer life than does the Euro 5w40 (probably not an issue for most Vette owners).

I do recommend that anyone wanting to use AMSOIL in their C7 follow AMSOIL's guidelines and use the Sig Series 0w40 for one main reason - warranty. Independent of the manufacturer warranty, the AMSOIL Guarantee covers products when used as recommended by AMSOIL, in this case the Sig Series 0w40.

Differential Fluid
Almost as complicated discussion as the engine oil. The bottom line up front is that AMSOIL does not have a differential fluid recommendation for the C7 (or C6), thus for those concerned for warranty issue as discussed above, I would recommend you stay with the factory fill. For those willing to roll the warranty coverage dice or for those who's cars are out of warranty, the best AMSOIL choice is what I've been using in my 02 Z06 at the track for 15+ years:
AMSOIL Severe Gear 75w90
AMSOIL Slip Lock Additive

The history. Back when the C5 came out, the factory fill was SAE 75w90 Synthetic Axle Lubricant (GM part 12378261) and 4 oz of Limited Slip Axle Lubricant Additive (GM part 1052358). For the C5, AMSOIL recommends the Severe Gear 75w90 which does have limited slip additive in it. When the C6 came out, GM increased the pressure on the diff clutch plates from ~75 lb-ft in the C5 to ~120 lb-ft for better performance. The diff fluid recommendation remained the same as that for the C5. Almost immediately, C6 owners started reporting diff chatter and GM had many TSBs on this issue (search for "diff chatter" in the C6 tech section for more than you would care to read). After a few years, GM released a new diff fluid, Dexron LS 75w90. This fluid has a LOT of additive in it to deal with the increased diff clutch pressure and to try to prevent the diff chatter that bothered so many C6 owners. Though I asked AMSOIL to list the Severe Gear 75w90 + 4 oz of Slip Lock Additive for C6 differentials on their product application guide, they never did. With the C6 out of warranty, I do recommend the Severe Gear 75w90 with 4 oz of Slip Lock Additive to C6 owners (and I have C6 diff internals in my diff). Along with the AMSOIL Torque Drive ATF for the C6 (and C5) manual (also what I use in my tranny which has C6 internals in it), the Severe Gear 75w90 is what I get asked most for in PMs and emails.

As to whether the Severe Gear 75w90 needs the additive or not, the answer is also a complicated - it depends. The short story here is that I recommend (and use myself) the Slip Lock Additive in all cases except those whose primary use of the Vette is at the drag strip and maybe auto-x. While diff chatter really bothers many C6 owners in daily driving, diff chatter won't lead to immediate diff failure. When I started racing my Vette, I didn't use the additive and raced without it until 2013 when I upgraded my diff to C6 internals (e.g. increased diff clutch pressure to ~120 lb-ft). Theoretically, this increased pressure should result in improved performance on the track coming out of slow speed hair pin corners. What I immediately noticed is that instead, it led to increased high speed understeer (push). I might not have put two and two together but had been reading a lot of about e-diffs at the time which open at high speed for this very reason; the increased diff clutch pressure results in dragging the inner rear wheel more through a corner to work against that diff clutch pressure given the different arcs of the inside (shorter) and outside (farther) rear tires. Thus a mechanical diff is a compromise between low speed performance (you want more lock coming out of a hairpin) and high speed performance (you want almost completely open diff). E-dffs can optimize both. Given the sticky tires I use on the track, the low speed performance isn't all that important as my tires maintain grip pretty well and I don't get inner tire wheel spin. To help balance the performance, I started using the additive and have been for the last 5+ years. So, for the street or track I think the additive is the right way to go. For drag strip, maybe auto-x, where you want maximum performance, I would go without the additive. Probably doesn't matter for e-diff.

Synthetic Basestocks
Addressing the comments above on whether the AMSOIL OE (and XL) lines are or are not "full synthetic", like all the above discussion, the answer is complicated. The main issue is that like many things (organic food for example) the term "synthetic" is not defined by any specification body, thus it comes down to marketing oversight.

AMSOIL was the first to develop a synthetic oil to meet API specs in 1972 - they didn't invent synthetic, just saw the opportunity for automotive applications. Mobil 1 and Castrol followed them to the market. In the early days of synthetics, all three either use a polyol ester synthetic basestock, called Group V by the API, or a polyalphaolefins (PAO) basestock, called Group IV by the API, or a blend of the two. Meanwhile, Chevron had been further refining mineral oil and getting better and better performance out of mineral oil basestocks. Not as good as Group IV or V, but better than conventional oil (called Groups I and II by the API). The API calls this highly refined mineral oil Group III.

In the 90s, Castrol changed their basestock to Group III and continue to label it "synthetic". Given that Group III basestock is less expensive than Group IV or V, this increased Castrol's profit margin. Mobil "sued" Castrol for false advertising... and lost. Thus today, in the U.S., Group III, highly refined mineral oil, can and is labeled "synthetic".

For about 15 years, Mobil hung on and advertised that their oil would remain a Group IV PAO basestock. About 2007, their advertising changed and now, like almost everyone else, their basestock is predominantly Group III (based on oil analysis I have seen on the oil forums). Mobil will not admit this and their website is non-committal saying their basestock "includes PAO" but not in what percentage: https://mobiloil.com/en/faq/ask-our-...ly-mineral-oil

To remain competitive, AMSOIL followed suit and now makes two oil lines (XL and OE) with Group III basestock. But, like Mobil 1, even AMSOIL stopped advertising what was or wasn't in their basestock about five years ago or so. As I watch the specs closely and haven't seen any change in performance of their top tier street and racing oils (and tranny and diff fluids), I firmly believe that they remain Group IV/V blends but I have no "proof" of this. So, whether the AMSOIL OE/XL lines (or almost any other "off the shelf" oil on the market in the U.S. today), whether it is or isn't "synthetic" comes down to your personal definition of what synthetic means. Here in the U.S., the current ruling is Group III is synthetic.
Cheers,
Brad
P.S. Going on travel this week and won't likely get back to this thread until next weekend, but will try to keep up with emails and PMs.