every country has these seasonal changes (except those near equator) both for gasoline diesel and lpg and this is before we get into renewable components

gasoline and lpg are changed for the same reason, it is to keep vapor pressure at the same temperature lower in summer and higher in winter. too low and it’s hard to start, too high and volatile components bubble out = gasoline degrades and makes smog in the city. diesel is more complicated because it gels out at lower temperatures but how exactly this happens depending on several things, and mitigations are different