It's not about years it's about air hours or flights. At 1 flight per year, an engine will last... forever??.
The OS 140RX is NOT a longstroke engine. OS went back to the 'square' configuration (bore = stroke). Vibration levels went up in the longstroke engines. Read the review in RCM magazine.

As for the original OS 61RF-P in question: what plug are you using and what does it look like after these short flights? That engine will turn a 12x10 APC very happily with a pipe length of about 16.5" measured from the glo-plug to the 'high point' or first baffle of the pipe.
How hot was the engine when you landed? Overheating will kill the liner in the 'ABC' (really ABN) OS engines by causing it to peel [:'(] .
Avoid castor oil. Set the engine up to run slightlly rich. I use K&B 1L glo-plugs and get 60+ flight per plug with no element distortion. If the glo-plug element is not a nice smooth spiral (or missing altogether) after flying then the engine is running too hot due to a lean run or the pipe set too short.
As for castor oil... I have more than 7000 recorded flights with Hanno specials: Castor oil just builds up on the piston and sleeve making the engine run hotter. I know guys swear by it, but the quality of current synthetic oils eliminated the need for it decades ago.