On Sunday, Day 1 of service, 991 used a mixture of A22 Euro 6s and 1 A22(Euro 5), however, on 10 July, this service was observed to be operated exclusively by the brand new MAN A22 Euro 6s which had just been leased to SMRT and 991 was their first revenue service in the SMRT network. The MAN A22 Euro 6 was exclusive to SBS Transit and only made rounds on 851e as there were so few buses but with more being delivered, MAN A22 Euro 6s have been observed on 851 and the newest examples have been leased out by LTA to SMRT for use on this new service.
As it served multiple estates, specifically the new ones in Bukit Batok West, this bus was relatively crowded. However, as it was only day 3 of operations, passengers and even drivers were unfamiliar with the route, and hilarity ensued when some aunties boarded opposite my church( this bus stop was served by 941 only, until 991 was introduced and 941 was diverted away) and went one round around the new estates only to loop back to the new bus stop outside my church
However, the current routing of 991 is only temporary, it has been announced that by Q4 2018 991 will be alterrd from a loop service to a truck service, operating between Bukit Batok Interchange and Choa Chu Kang interchange. This change is likely to coincide with the opening of the new Choa Chu Kang bus interchange. As usual, I Vlogged the entire route of 991 and I will post the link to the video here as soon as I can edit it to give it location tags.