At this point, we have bantered about quite a bit regarding properties of various materials. Now might be a good time to get back to the original basic question of which 10mm bolts the original poster might want to use for his engine mounts.
From where I’m sitting, Alloy 20 or Titanium might be a nice choice if your budget allows & if you can find them. It looks to me like if you want to get them, you need to ask a manufacturer for a quote, so I’m going to take those choices off the table on the basis of poor availability.
That would leave me choosing between the classic materials that can be found on the shelf, like 316 (aka A-4), 304 (aka 18-8 or A-2), bronze, brass, 8.8 or 10.9 or 12.9 grade steel. The steel options would most likely be plain or cadmium plated, or zinc plated. If steel was used, I’m thinking that you would either want to paint the bolts after installation or smear them for 100% coverage with grease.
So then, who votes for which options?