I did read the reseach paper by Faculty of Engineering, Khon Kaen University, Thailand. The link is in previous post.

The engine they run failed on 64th hour continuous running after putting 15% extra load at 50th hour run. The deposition at inlet valve cause the engine to stop. Piston, block and piston ring is okay.

So means that cooking oil will damage the engine head due to clogging at head. So you need frequent change your 'engine oil' to avoid this problem, or put some engine oil additives.