At the time, I'd assumed there was something lodged in the rear crankcase but didn't investigate further. Some three years or so later, while preparing to clean out the rear engine's oil sump, the actual reason has become apparent.
Take a look at the following photo with the two dipstick heads highlighted.
|A new slant on dipsticks|
This answers the mystery of the differing oil requirements for each engine. What worries me is how long has it been like this and why are the two engines different?
Of course, the rear engine will have been running with two fewer gallons of crankcase oil than the front one. Hmm...
At some time in the future, the rear dipstick will need fixing. In the meantime, yet another mystery has also been solved.
In this photo, there is a drain cock to let water out of the sump and another highlighted in the top right.
|Extra drain cock, top right|
The rear engine had a blanking plug instead of a drain cock so I swapped the two. The rear engine oil level can now be checked using the drain cock while the front engine's dipstick can still be relied upon.
The mystery has been solved but why the two engines are not identical remains an unknown.