hi am i right in thinking the only difference between the 328P-20PU and the 328P-PU is the 20PU is good for 20Mhz? as apposed to the 16Mhz 328P-PU??
Different suppliers may pick a different name (sometimes even incorrect or misleading) for their product listing.
As far as I know there is no 16MHz DIP version of the AtMega328 as all DIP versions are rated for 20 MHz (there is an automotive version rated for 16 MHz max, but they do not come as –PU versions). The bootloader will require/expect a specific frequency and so you need to verify that this match your requirements.
As for 328P-PU versus 328P-20PU I think it is safe to assume they’re not only compatible, but identical.