* May WCS quoted at $14.75/bbl under WTI
* Flint Hills, Exxon, Suncor conduct refinery upkeep
* May light synthetic quoted at $5.25/bbl over WTI
CALGARY, Alberta, April 16 (Reuters) - Canadian heavy crude prices weakened on Tuesday as some large refineries in Minnesota, Illinois and Alberta conducted maintenance, reducing demand.
Western Canada Select heavy blend for May delivery last sold for $14.75 a barrel below benchmark West Texas Intermediate, compared with a Monday settlement of $14.40 under, according to Shorcan Energy Brokers.
The spread has gradually widened from $11.90 under WTI earlier this month, but is still a far cry from January, when WCS differentials surpassed $40 a barrel under amid surging production and limited export pipeline capacity.
This week, Flint Hills Resources started planned maintenance of a coker unit at its 320,000 bpd Pine Bend refinery in Minnesota, according to Genscape.
The work comes as Exxon Mobil Corp began a major turnaround at its 238,600 bpd plant in Joliet, Illinois.
Last week, Suncor Energy Inc started planned maintenance at its 140,000 barrel per day refinery at Edmonton, Alberta. The plant relies on heavy crudes from the northern Alberta oil sands for its feedstock.
Light synthetic crude for May delivery last traded at a premium of $5.25 a barrel above WTI, flat with Monday.
The premium has narrowed as much as $10 in early April, which put it nearly on par with international benchmark Brent crude.
"The arbs were closed so it had to fall off a bit," a trader said.
Synthetic crude has strengthened from the winter's lows as planned maintenance at oil sands upgraders owned by Suncor and Canadian Natural Resources Ltd is expected to cut into supply in May.