For the motor on a 1300 a 228.3 oil should be sufficient. 228.5s are synthetic more or less and meet more stringent test requirements.
A 300 series motor (yours) can actually use a 228.1 oil, as can my 906 since it is only Euro III.
More specifically, 228.1 meets the old ACEA E2, 228.3 meets old ACEA E3 and 228.5 meets current ACEA E4, all with some additional MB tests. SOME but not all 228.3s also meet current ACEA E7, with some additional tests.
Unimog U500 with Unicat camper; diesel BMW X5 35d, diesel BJ40 Landcruiser and diesel M37