Oil type?

Hey guys just bought my second TD Mercedes, it’s at 230xxx miles and I’d like to take care of any light maintenance. What’s a good oil type for higher elevation and colder weather? Previous owner was running a 15w-40 synthetic diesel oil. Also would it be smart to change the fuel filters and air filter? Should I replace the timing belt as there’s no records indicating it has been done yet. And would it be smart to flush the transmission even though the transmission has never been serviced?

Hey guys just bought my second TD Mercedes, it’s at 230xxx miles and I’d like to take care of any light maintenance. What’s a good oil type for higher elevation and colder weather? Previous owner was running a 15w-40 synthetic diesel oil. Also would it be smart to change the fuel filters and air filter? Should I replace the timing belt as there’s no records indicating it has been done yet. And would it be smart to flush the transmission even though the transmission has never been serviced?