The Santa Monica oil-refining and marketing company said the new pact would give it greater liquidity. It relieves Tosco of paying interest on its debt this year, unless cash flow exceeds specified levels, deferring the unpaid portions to January, 1987. It allows interest payments already made this year to be applied to repayment of principal instead. It sets specified dates by which the company must pay parts of its $40 million that it still owes. Also, it gives the company an extra $10-million line of credit and extends a current $15 million in credit.