My uncle bought a really cool old military bike. The "OWNERS MANUAL IT SEZ....."HQ-10 FOR SUMMER & HQ-6 FOR WINTER.* DON'T HAVE A CLUE WHAT THE HELL THAT MEANS IN U.S. OIL VISCOSITY."

Anyone here know that verbiage?

 

HQ-10 would be equivalent to 10w30 or 15w40 in today's oils. I imagine HQ-6 is a lightweight oil much like #6 diesel oil. So I would try using 10w30 or 15w40 Shell Rotella in it. HQ-10 is only referenced in machine lubricating oils today. Do NOT use synthetic oil in that engine. In Cali 10w30 should be fine all year. BTW, if it has a wet clutch then use the 15w40 Rotella as it is JASO certified for wet clutch applications.