What are some of the changes Magnus?
Ignition system, fuel systems, engines combustion design / temperatures as well as fuels used to mix are all different.
I call these newer type engines fresh air engines as they get air in without oil, fuel that lubricate and cool.
Older oils have a hard time handling the heat from combustion as it is hotter and often burn longer time. This is to get a better emission reading so our environmentalists can sleep well on the plane to Bermuda for vacation a weekend.
Heat that is higher than the oil is meant to use cause it to carbonize before its done its done doing its job. Some times before its done any of its jobs.
Often this gets stuck inside engine on walls of ports, piston head, in case and on crank/bearings. The carbon buildup prevent cooling and lubrication, restrict flows and that is not good. Sometimes it sticks to pistons skirts as this is already too hot for the oil as it goes in to engine. This create friction = more heat.
You can see this when you tear them down after they gone caboom. Burned carbonized oil almost every were.
I often see this when they run wrong oils in older saws. When they broke they put them under bench and now 20-30 years after they bring it here for me to play with.
I have customers come with a bunch of 80's saws in boxes that did their job then back in the day. They ask me to get what I can going again so they can get their hands on something that work more than a couple weeks at the time.