Replacement Tires and Load Ratings

BGTV8

Do not worry about the XL rating ......... what you need to work off is the load index which is the 2 or 3 digit code followed by an alpha after the size.

The Z34 requires 95W front and 96W rear "load rating".

My 2009 with 18" wheels has 225/50R18 95W and 245/45R18 96W as standard.

An "XL" tyre simply says that the load is influenced by tyre pressure.

For an explanation of "load index" - see here:
Tyre Markings - Bridgestone Australia

The "XL" designation is explained here:
Web theft yields the following...

Extra Load tyres are designed to be used at higher pressures than normal tyres. They have the same maximum load capacity as a normal tyre with the same Load Index but achieve this at a higher pressure. Normal tyres are inflated to pressures between 2 bar and 2.5 bar. They reach the maximum load capacity as specified by the Load Index when inflated to 2.5 bar (35psi) Eg for a Load Index of 94 this is 670kg. Extra Load tyres can be inflated to pressures up to 2.9 bar (41psi) and actually have a lower load capacity at the same pressure as a normal tyre with the same Load Index. To get the 670kg load capacity for a Load Index of 94, an Extra Load tyre must be inflated to 2.9 bar. At 2.5bar it will only have a load capacity of 595kg

So, and XL tyre is actually less capable that a non-XL tyre as they achieve compliance with their actual load index at maximum permitted pressure.