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First of all, a review.

x = 0

I would give a more meaningful name.

year +=1

PEP 8 says:

Always surround these binary operators with a single space on either side: assignment (=), augmented assignment (+=,-= etc), comparisons (==,<,>,!=,<>,<=,>=,in,not in,is,is not), Booleans (and,or,not).

(Emphasis mine)

if ...:

    ...
    year += 1
else:
    ...
    year += 1

If you are doing the same thing regardless of the if statement, move that line after the if and else.

print("%s is a leap year") % (year)

% format strings are not officially deprecated, but it is recommended to use .format()

Now for the better ways.

There's a fancy little function that was made for checking leap years: calendar.isleap

from calendar import isleap

def loop_year(year):
    found = 0
    while found < 20:
        if isleap(year):
            print("{} is a leap year").format(year)
            found += 1
        else:
            print("{} is a common year").format(year)
        year += 1

If you don't care about the common years (based on your commenting out that line), remove the else:

from calendar import isleap

def loop_year(year):
    found = 0
    while found < 20:
        if isleap(year):
            print("{} is a leap year").format(year)
            found += 1
        year += 1