For an '89 300 E: check & correct automatic transmission fluid level every 15K miles; renew trans. oil and filter every 30K miles. Use M-B approved Dexron transmission oil (presently available as Dexron III). If the transmission is in good condition, then changing the oil more frequently will cause no harm.
The torque converter on your vehicle is equipped with a drain plug; when renewing your transmission oil ensure that the converter is also drained (as it holds approx 2.1 qts).