tire pressure with filled tires

[Texasmark]

I have a question about the tires on my larger Kubota tractor. They are filled from the dealer and I have never added any air to them the whole time I have had the tractor since they look and feel fine. With the fluid in them would I inflate them to the same pressure rating that it says on the tire? I know I have to get the valve stems up to when I check so the fluid doesn't spew everywhere (probably why I've never messed with them before).

I run the 6530 with filled tires
50 gal per. 16.9x28 6 ply, tubeless, bias. Tire says 18 psig for max rating. I run about 10 for rider comfort. Had the tractor 10 years now and have just recently checked air pressure and still ok. A little soft on the right when I have the 6' drum hanging off to the side, but no biggie.