Vaseline won't ruin your silicone toys, but it doesn't lube well and is too thick to be very effective. ToyTimeTim is also right about it being hard to clean off. Water based lubes aren't very expensive and work well for almost any sex use, and commercial oil lubes can also be a good choice for silicone.
Unlike oil-based lubes specifically designed for anal sex, vaseline lacks the correct viscosity (or thickness), for smooth lubrication. If not cleaned out well enough (and "I got almost all of it off" will not suffice here), then being that it tends to harbour bacteria and is extremely hard to wash off completely, you are risking leaving the bacteria collect thereby increasing the risk of contracting an infection.
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Vaginal sex is a different story, though, as the natural lubrication should have taken over by the time water and silicone based lubricants are starting to wear out. Another benefit of vaseline comes when you are using silicone toys. Water based lubricants and oil based lubricants (such as vaseline) will not dissolve silicone toys.
Its creators neither designed nor marketed Vaseline as a sexual lubricant. Vaseline can damage condoms. People planning to use latex condoms should not use Vaseline as a lubricant.
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It’s understandable why Vaseline or petroleum jelly is often seen as a viable alternative to store-bought lube and sex-specific products. Vaseline is affordable, just like most petroleum jelly products, and it has a thick consistency.
People might want to use Vaseline as a sexual lubricant because it has a soft and smooth texture. Vaseline does not get sticky or dry out, which — in theory — also makes it a good lubricant option. However, it is not advisable to use Vaseline as a sexual lubricant.
Fleshlights aren't 100% silicone they are some sort of blend. I've noticed over a couple years that using vaseline SEEMS to me to damage the fleshlight by forming tiny tears, but not other silicone toys. Perhaps the fleshlight is just worn with use and it's not the vaseline. In any case, fleshlights aren't as durable as other toys in my opinion.
Vaseline is generally regarded as very safe for external use. There is one study that suggests petroleum jelly might be associated with an increased risk of bacterial vaginosis and yeast infection, but stops short of finding a causal link.