I completed my work. I thought I should update this thread in case anyone searches for the same information. By holding the old pads to a fine ruler, I estimated their thickness at 1mm. I can't be sure if they may have permanently compressed over the years, but the blue pads did rebound fairly quickly when squeezed. Anyway, I have no other information, as Dell didn't record pad thickness or type in the Service Manual or on internal documents that still exist. I bought NAB Cooling thermal pad, which did well in testing according to online magazines. It comes as a single sheet with removable plastic film on both sides. I measured the sizes that I needed, marked cutting points on the film, then put the pad in the freezer for half-an hour. Chilling it firms it up, making it a little easier to cut cleanly. As I don't have a proper art cutting mat, I used a kitchen wooden cutting board and a fine utility knife. The consistency of the NAB Cooling pads is close to that of the denser dark grey pad that originally covered the Intel HM67 chipset. It feels quite different from the squishy blue thermal pads that covered VRAM and other chips around the Nvidia GPU. Some reviews compared the NAB Cooling pads to Gelids, both in consistency and performance. I cleaned the laptop, removing a fair amount of accumulated dust. I disassembled the fan to thoroughly clean out dust and fluff. The fan now looks like new. I used ArctiClean 1 to remove all traces of original thermal compound and pad residue. (As I've seen often when opening up laptops, the original installer used too much thermal compound.) I used ArctiClean 2 to prepare surfaces for new thermal compound and pads.  I applied Arctic MX-4 thermal compound to CPU and GPU, and applied the thermal pads to components that originally had them. The laptop is back together. I ran Memtest86+ for a few passes with all CPUs in parallel mode, to stress it. Temperatures are good.  I wish Dell's service manuals would list the type and thickness of thermal pads. That would help users and technicians when working on these units after Dell stops supplying its own contractors with pad kits (probably only 3 or 4 years, until warranties run out). Some Alienware models have multiple pad thicknesses, all the way from 0.5mm to 2.5mm in a single laptop. Just like screw sizes, it would be good to have all this information in the service manuals.
I realize this is a long shot, but I thought I'd ask here before buying something. I'm cleaning and repasting my 11-year-old XPS 15 L502x laptop (2nd-gen Core i7, Nvidia GeForce GT 525M), model from 2011. As you can see in the photos, there are several original thermal pads. Light blue thermal pads cover VRAM and other chips around the Nvidia GPU (blue highlight circles), while a denser dark grey pad covers the Intel HM67 chipset (red highlight circle). I want to know: Regarding replacing thermal pads, the Service Manual says: "use the thermal pad provided in the kit". Presumably, licensed technicians were provided with an XPS repair kit that included appropriate thermal pads, so specs weren't listed. XPS and Alienware owners posted that if they chose the wrong thickness, the heat sink didn't make good contact with the paste on the GPU. I've put the pads against a ruler and they appear to be 1mm thick, though it's possible that they permanently compressed over time. Unless anyone here knows what was specced, I'll choose 1mm pads. Unfortunately, I'll need to buy a full sheet or package, though I don't need much. [IMAGE] [IMAGE]
Thermal pad thickness for 11-year-old XPS 15 L502x