The recommended oil for all airheads is 20W-50 but here’s a thought.
All airheads from R45 to R100 have much the same oil capacity and cooling surface area give or take one or two barrel fins. I’m talking standard naked bikes with no oil cooler or deep sump. It would be no surprise then if an R100 ran hotter than an R45 with other models in between.
It’s my experience that an R45 does run cool. I think maybe much too cool. I’ve measured the sump temperature several times with a calibrated digital thermometer and it’s typically 82 Deg C after a 40 mile run on country lanes max. speed 60 mph - two up though and that works an R45 reasonably hard! Shorter runs in the winter produce lower sump temperatures.
Lets say for the sake of comparison that under similar modest circumstances an R100 is running 20W-50 oil at 88 Deg C and an R45 is running 10W-40 oil at 82 Deg C. A graph reproduced from an oil companies data shows that they are both running oil of the same viscosity ie 40Cst. My point is the engines will not know that they are using oils of different basic specification.
I’m not saying that an R100 typically runs at 88Deg C. I’m just trying to illustrate that it doesn’t take a much lower operating temperature for 10W-40 to be the same viscosity as 20W-50 . The temperature difference may well be much greater or the R100 would be running too cool as well.
So have I made a case for an R45 to safely run 10W-40 with the benefits of easier cold weather starting, quicker oil circulation and lower oil pressure from cold or am I missing something fundamental like much higher temperatures at bearing surfaces with implications for oil film strength ?
