Canada crude -Heavy grades retreat from recent highs

* March WCS trades at $21.00/bbl below WTI

* March synthetic trades at $1.50/bbl below WTI

CALGARY, Alberta, Feb 5 (Reuters) - Canadian cash crude prices fell on Wednesday, with some traders saying recent gains had gone too far given the looming refinery maintenance season that will likely crimp demand.

Western Canada Select heavy blend for March delivery last traded at $21.00 per barrel below the West Texas Intermediate benchmark, according to Shorcan Energy brokers.

That compares with a settlement price on Tuesday of $19.35 per barrel below the benchmark, and marked a drop from Monday's six-month high settlement of $18.05 per barrel below the benchmark.

"Going into turnaround (season), it's often weaker at this time of year," said one Calgary crude trader, who also noted heavy crude prices were much stronger than this time in 2013 when they traded around $30 per barrel below WTI.

Analysts have in part attributed more resilient prices this year to a relatively light maintenance turnaround schedule, which will get underway towards the end of this month.

Despite the maintenance, demand is likely to remain healthy with BP Plc (LSE: BP.L - news) 's 405,000 barrel per day Whiting, Indiana, refinery switching over to processing more heavy Canadian crude.

Citgo Petroleum Corp also said the vacuum distillation unit at its 174,500 bpd Lemont, Illinois, refinery was restarted on Tuesday and was at full production capacity after being shut since an Oct. 23 fire.

Light (Other OTC: LGSXY - news) synthetic crude from the oil sands for March delivery also fell to $1.50 per barrel below WTI, compared with a settlement price on Tuesday of 25 cents below the benchmark.