Why new thermal paste is making my laptop hot about 90C

HY-510 is low grade bulk thermal paste. More than likely if you've connected the CPU fan and applied it properly then the paste is of poor quality and the processors from this era were also known to run hot, so cheap thermal paste was known to cause temps to spike. That's why we repasted so many of these laptops back then the second the warranty expired 1 week later (just to play it safe. We knew it wasn't an issue to break it open to scrap that trash on day -1, but +6 days on that won't kill you).

These days it's less of an issue as the processors are sometimes as low as a 15W TDP part (35W laptops like yours still exist). The difference today is unlike the old days the modern 35W laptops use good thermal paste and better cooling as the OEM's build out these high-end machines since the bulk of modern laptops use 15W "ULV" processors which they can put 15W of capacity and a buffer in (they might use a 15W chip, but enough copper for ~25-30W). Today, the people who buy the current generation of "35W TDP" laptops will pay for them now as it's become a premiun option with "15W ULV" laptops being everywhere, so they give actual thought to the build since the market wants a better machine then every Acer laptop with a ULV chip. Business class Ultrabooks have always been well built at the premium end, so this level of quality is new to the average user; we've had this on our Latitudes and EliteBooks for 10+ years. Welcome to our world; now you see why those of us who use business class notebooks swear at plastic machines.

As far as the materials now, think premium materials like aluminum chassis being largely STANDARD, with mostly copper heatsinks. These newer high power laptops do not have a copper pipe and an aluminum supporting plate around the small amount of copper they used for CPU contact like your T430. Your T430 is from the time when ALL of them had cheap thermal paste and questionable cooling, so it will run hotter than you may want, but 90C is too high for a T430 -- even the (cooler running) T420 wasn't this bad.

Put a good paste on like Artic MX-5. It works very well on both 15W machines, and 35W ones in general.