As requested by some laymen members, here is the SBS Transit fleetlist of all the buses we are talking about(only fleet in operation will be included)
SBS 31L - SBS 80UScania N113CRB original air-con buses. Found mainly in East district and Toa Payoh. Buses were introduced in 1989. Bodywork by Walter Alexander of the UK.
Engine: Scania DSC1171
Gearbox: VOITH D381.4
SBS 81SMAN 16.240. Was introduced in late 1989 and is now used as a training bus. Bodywork by PSV of Holland.
Engine: MAN UH-series
Gearbox: ZF 5HP500
SBS 82P - SBS 281GMercedes Benz O405. Launched in 1990. These buses can be seen at Central district and West district. Bodywork by Walter Alexander.
Engine : Mercedes-Benz OM447h
Gearbox: Mercedes-Benz W3E.O80
SBS 282D - SBS 581RMercedes Benz O405. Launched 1991-1992. Buses can be seen at Central district and West district as well. Bodywork by Duple Metsec of the UK.
Engine : Mercedes-Benz OM447h
Gearbox: Mercedes-Benz W3E.O80
SBS 582M - SBS 881AVolvo B10M Mk III. Launched 1993. Buses can be seen in East and West districts. Bodywork by Duple Metsec
Engine: Volvo THD101GC
Gearbox: VOITH D381.4
SBS 882Y - SBS 996CVolvo B10M Mk IV. Launched in 1995. Buses can be seen mainly in East district. Bodywork by Duple Metsec.
Engine: Volvo THD101GC
Gearbox: ZF 4HP500
SBS 997AVolvo B10M-70 Mk IV. This is the superlong bus. Launched in 1995 and is on Sv. 62. Bodywork by Duple Metsec.
Engine: Volvo THD101GC
Gearbox: ZF 4HP500
SBS 998YVolvo B10M-60 Articulated. Also a Mk IV. This is the first articulated bus in Singapore. It is currently on Sv. 18 with bodywork by Duple Metsec. Launched in 1996.
Engine: Volvo THD101GC
Gearbox: ZF 5HP500
SBS 999UMercedes Benz O405G Articulated. Second articulated bus in Singapore. Launched in 1997/1998. Bodywork by Volgren of Australia. Currently on Sv. 18
Engine: Mercedes-Benz OM447hA
Gearbox: ZF 5HP500
SBS 1688KSingapore's first Volvo B10BLE low-floor bus. Launched in 1997 and is currently on Sv. 85, with bodywork by Volgren.
Engine: Volvo DH10A
Gearbox: ZF 5HP500
SBS 1696L - SBS 1895DVolvo B10M Mk II. The bodywork varies by registration number. The buses were launched in 1988-1989. Most were converted to air-con. Bodywork by Duple Metsec and Alexander.
Engine: Volvo THD101GC
Gearbox: Volvo 3-speed
SBS 1881T / SBS 1888AThese two buses are Volvo B10M Mk IIs and they are special in that sense they have an unconventional under-floor air-conditioning system. However the exhaust pod for SBS 1888A air-con system is on the roof of the bus. Both were launched 1988-1989 and have Duple Metsec body.
Engine: Volvo THD101GC
Gearbox: Volvo 3-speed
SBS 1896B - SBS 1999MVolvo B10M Mk IV buses with Alexander Strider body. Introduced in 1996.
Engine: Volvo THD102KF
Gearbox: ZF 5HP500
SBS 2593T - SBS 2688DSame as above.
Engine: Volvo THD102KF
Gearbox: ZF 5HP500
SBS 2689B - SBS 2838MVolvo B10M Mk IV buses with Duple Metsec DM3500 body. Introduced in 1997/1998.
Engine: Volvo THD102KF
Gearbox: VOITH D863.3
SBS 2888TSBST's first Scania L94UB low-floor bus. Launched in 1999 and now on Express 518. Bodywork by Volgren.
Engine: Scania DSC912
Gearbox: ZF 5HP500
SBS 2988M - SBS 2989GSingapore's first B10BLE CNG bus. Only 2988M and 2989K in service on Jurong Island services. Bodywork by Volgren. Launched 2002.
Engine: Volvo GH10B 245
Gearbox: ZF 5HP500
SBS 3550K - SBS 3571ALeyland Olympian 10.6m double-deck. Will be phasing out soon due to lack of air-con and age.

Bodywork by Alexander. Launched in 1986-1988.
Engine: Leyland TL11
Gearbox: Allison World Transmission 4-speed
SBS 3572YSingapore's sole Leyland Lynx that was introduced in 1989. Bodywork is by Alexander and is now an employee bus.
Engine: Cummins L10A
Gearbox: ZF 5HP500
SBS 3573U - SBS 3722GConverted air-con Scania N113CRB buses. Launched in same timeframe as original aircon buses. Bodywork is the same and they were converted to aircon in 1999. However the older series of this bus are still non-aircon.
Engine: Scania DSC1104
Gearbox: VOITH D381.4
SBS 3723D - SBS 3922XConverted aircon Mercedes Benz O405 buses. Several SBS 37xx buses were reregistered to PA rego and assigned Jurong Island services. All have Duple Metsec body and all were converted to aircon.
Engine: Mercedes-Benz OM447h
Gearbox: Mercedes-Benz W3E.O80
SBS 6909R - SBS 6999ELeyland Olympian 10.3m non-aircon double-deck. Commentary above. SBS 6909R is now the Kingsmen bus located at the Youth Park.
Engine: Leyland TL11
Gearbox: Allison World Transmission 4-speed
SBS 7113M - SBS 7197XSame as above.
Engine: Leyland TL11
Gearbox: Allison World Transmission 4-speed
SBS 7198E - SBS 7297RVolvo Olympian 10.3m non-aircon double-deck buses. Launched in 1994. Last non-aircon buses in SBST fleet. Bodywork by Alexander.
Engine: Volvo TD102KF
Gearbox: VOITH D851.2
SBS 8009A - SBS 8018ZDennis Dart 9.4m midi-bus. Serves Sv. 42 and Sv. 92. Introduced 1994-1995. Bodywork by Duple Metsec.
Engine: Cummins 6BT
Gearbox: Allison AT545
SBS 9000S - SBS 9199CLeyland Olympian 12m Superbus. SBS 9000S is the first air-con double-deck. Bodywork by Alexander and they were launched in 1993. This batch of Leyland buses are the last in the world.
Engine: Cummins LT10
Gearbox: ZF 4HP500
SBS 9200G - SBS 9400XFirst generation Volvo Olympian Superbus. Lanuched in 1993-1994 with same bodywork as the above.
Engine: Volvo TD102KF
Gearbox: ZF 4HP500
SBS 9401T - SBS 9670HSecond generation Volvo Olympian. Launched 1996-2000. Bodywork is similar to that of above but by same company.
Engine: Volvo DH10A
Gearbox: ZF 4HP500
SBS 9671E - SBS 9690ADennis Trident. It was launched in 2001 and it has Duple Metsec body. It was rather advanced in that year with all the new systems like EDS and low-floor.
Engine: Cummins M11
Gearbox: VOITH D863.3
SBS 9800A - SBS 9849KVolvo Super Olympian low-floor Superbus. Launched in 2002 and has Volgren body. However SBS 9835A onwards has not been seen on the road yet.
Engine: Volvo DH10A
Gearbox: VOITH D863.3
SBS 9888YSingapore's first Volvo Super Olympian. It was launched in 1999 and has Volgren body. Currently on Express 502.
Engine: Volvo DH10A
Gearbox: VOITH D863.3
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Well, that's all, any comments can be made here, while corrections should be e-mailed to me so I can edit my post.
Thanks to Windy for the engine and gearbox corrections.