trigger AUTOADDPRODUCT on OpportunityLineItem (after insert) {
    List<OpportunityLineItem> OpportunityLineItems = new List<OpportunityLineItem>();
    for (OpportunityLineItem newOpportunityLineItems: Trigger.New) {
        if(newOpportunityLineItems.Name == 'CarOIL'){
            OpportunityLineItems.add(new OpportunityLineItem(PricebookEntryId ='ID CODE FOR CarOILplus',Quantity = 1,UnitPrice = 25));
        }
    }
    if(!OpportunityLineItems.isEmpty()){
        insert OpportunityLineItems;
    }
}