1987 TR. Oil pressure is only 35-38 on highway at 65-70 mph. Outside air temp was in the 60's. I use 10W30 Valvoline VR-1 with ZDDP. Used to be 50-55 psi on highway. I have about 60,000 miles on rebuilt motor. Pressure is 15-20 at hot idle. Could gauge be going bad, sender going bad, or could this be somethng serious? Thanks!